Creating a High-Powered Air Blaster

(Nerf Gun)

MIT Maker Portfolio

Created by: Daniel Foreman

Summer of 2015

Table of Contents

Summary: ...................................................................................................................................................... 1

Objectives: .................................................................................................................................................... 1

The Challenge: ............................................................................................................................................... 1

Design Choice: ............................................................................................................................................... 4

Gun Dynamics in More Detail ................................................................................................................... 5

Dead Space and Air Leakage ..................................................................................................................... 7

Creating a Gun Design ................................................................................................................................... 7

Determining the Gun Body Lengths .......................................................................................................... 7

Determining the Materials Needed .......................................................................................................... 7

Creating the Air Canister ........................................................................................................................... 8

Creating the Plunger ................................................................................................................................. 9

Determining the Spring ........................................................................................................................... 10

Determining the Barrel Length ............................................................................................................... 11

Determining the Dart Storage System .................................................................................................... 13

Determining the Trigger .......................................................................................................................... 14

Problems Encountered While Creating the Gun and their Solutions ......................................................... 15

Performance ............................................................................................................................................... 19

Accuracy .................................................................................................................................................. 19

Range ...................................................................................................................................................... 19

Darts ............................................................................................................................................................ 20

Dart Physics ............................................................................................................................................. 20

Alternative Darts ..................................................................................................................................... 21

Darts for the Future: ............................................................................................................................... 21

Thoughts on the Current Gun ..................................................................................................................... 22

Improvements for the Future ..................................................................................................................... 22

1

Summary: Often frustrated by inadequate performance (low range, low accuracy) of my commercial NERF guns, I

resolved to create a superior homemade alternative. To create this blaster from a large variety of

materials and limited tools/machining ability, the physics behind dart blaster was examined, past

designs were evaluated, both my own and from others, dart theory was investigated, and a design

sketch was created. The designs for both the trigger and the barrel-indexing method, to the best of my

knowledge, are unique to this gun and were created to make the operation of these historically

complicated parts simpler to construct and use. The construction of the gun was riddled with numerous

problems, many requiring creative or on-the-spot solutions. The fully constructed gun has room for

improvement but satisfied my design goals in both range and accuracy. Post-completion, the defective

dart-storage system and any broken pieces will be repaired or re-created. The entire creation of the

gun, from its conception to its post-construction testing, was a huge learning experience, particularly in

the importance and necessary flexibility of any engineering design and also unorthodox construction

techniques to compensate for my lack of precision tools.

Objectives: In order of importance:

1. Gun will shoot at least 70 feet (maximum diagonal distance across my lawn) flat

2. Gun must be able to fire darts moderately accurately (more accurately than a Nerf elite blaster)

3. Gun will be able to be usable in a nerf war

a. Must be able to fire multiple darts without needing to pause to reload

b. Must be able to fire a dart every 3 seconds

c. An dart-holding system which can work well in most typical conditions/firing angles

d. Gun must be able to be fired accurately without the need of a stand (portable)

The Challenge:

This was my first time successfully making a Nerf Gun. I have tried in the past two years to create one,

one from plans (FAR) and another one, a variant of another gun (SCAR-N). However, this year, I went

back to look through my design notebook and found the (very) early concept of this gun (fig. 1) and

remodeled it (fig. 2). Additionally, due to my lack of experience, everything in the gun needed to be

designed to be as simple as possible, lest I make a mistake and permanently damage the gun.

2

Figure 1. Original design inspiration for the gun, made 2 years ago.

3

Figure 2. Design sketches of various parts of the gun.

Top row left: Design for the gun body, plunger, canister, and trigger. The gun body design is on the center left.

Trigger designs are on the top.

Top row right: Design for the front of the gun and handles. The handle design is at the top of the page, left of

center. The design for the front of the gun is in the center of the page

Middle row left: Design for the barrels. The locations where barrels would be placed in the dart storage mechanism

(drum). Calculation of length on the right.

Middle row right: Design for the barrel indexing system. The design on the far right is the general shape of the

system. The circles are representative of the pins which would stick out of the drum (see center of the page). The

straight lines are the cam outlines. The cam would move forward and back as the gun is primed to index the drum

to the right.

Bottom row center: Design created during gun construction on how to attach handles to the gun.

4

While I do have many tools available to me, I do not have any benchtop tools and only have hand tools

and general power tools (fig. 3) so anything made had to be simple (especially when coupled with my

lack of experience)

Delivering air to a dart is not too difficult, and can be done with a simple pump, valve, and pipe easily.

However, giving the dart a good enough velocity to launch it 70 feet is slightly more difficult, although it

can be achieved in a very similar manner, a simple scaling of the above solution. However, achieving

decent range while retaining practicality (speed, weight/size/mobility, and reliability) is a far more

complicated task.

Design Choice: All Blasters work on the same principles (fig. 4), some mechanism stores the energy for the dart, an air

chamber (which I call a canister) which releases compressed air, and a barrel which holds the dart and

helps direct the air from the canister to accelerate the dart.

Figure 3. All my power tools.

From left to right: Drill, Vibrating cutting

tool, Small handheld rotary tool, Broken

Jigsaw

Figure 4. Inside a NERF Maverick pistol.

Green box-Mechanism to store potential energy (a spring

here)

Yellow box-Canister/Air chamber

Red box-Barrel

5

Each gun’s range (directly related to muzzle velocity) is dependent mainly the Impulse-Momentum

equation:

∆𝑃 = 𝐽

∆𝑉 = 𝑉𝑓 =𝐹𝑖𝑚𝑝𝑎𝑟𝑡𝑒𝑑 ∗ 𝑡𝑏𝑎𝑟𝑟𝑒𝑙

𝑚𝑑𝑎𝑟𝑡

From this equation, to create a higher muzzle velocity, more force can be added to the dart through the

use of more energy being stored or energy being transferred more efficiently, a longer time that that

particular force is being exerted on a dart, and by decreasing the dart’s mass. The gun itself has more

control over the imparted force and time the dart is being accelerated than over the mass of the dart.

There are two general types of air-powered blasters, a spring-loaded gun and a compressed air gun.

Spring loaded guns rely on the force of static friction to keep the dart in place and compress the air

behind the dart with a spring, launching the dart down a short barrel. This gun imparts more force per

unit time, but over a short period of time. These guns can be quickly reloaded and fired in quick

succession but have limited range. A second type of gun uses a large container of compressed air and a

long barrel to fire over very long distances, at the cost of an extremely slow firing rate and a relative

inefficiency, as the dart exits the barrel before the air can accelerate it to its maximum possible velocity.

Having a spring gun with a large air capacity and greater storage of energy (larger springs) will allow for

the creation of a fast, long-distance gun. Some guns, called HAMPS, exist, which have relatively high air

volumes and, with attachments, can be fired much faster than compressed-air guns (albeit, slower than

spring-loaded guns), although these guns can only be fired by human power, a disadvantage explained

in the following paragraph.

Gun Dynamics in More Detail The gun’s ability to fire the dart comes not its air volume, barrel length, or even stored energy per se,

but from its ability to accelerate the dart inside its barrel. Longer barrels are an obvious way to do this,

as, given the same firing force behind the dart, the dart will be inside the barrel for a longer period of

time, and thus be accelerated longer, producing a greater end velocity. High-volume air canisters exist

to be used with these barrels. As the dart progresses down the barrel, the force of the air behind the

dart lessens (Boyle’s law, volume in barrel-canister system is increasing and more than what it would be

with a static dart) so a larger air source is needed to lessen the impact of the decrease in volume created

by having a longer barrel. The second important part of the acceleration is the imparted force. In all

blasters, the static force of friction is overcome by the air pressure in the canister as the dart begins to

accelerate and the dart begins to fire. The force of static friction is much lower than the maximal force

the compressed air (regardless of the method) can supply since higher static frictions generally correlate

to higher kinetic frictions that render the gun useless as the dart would get stuck in the barrel and be

unable to fire. From there, the dart’s acceleration is determined by a combination of the dart and

plunger’s velocity. Higher pressure behind the dart will obviously make it accelerate faster, but

regardless of the pressure, once the dart exits the barrel, the acceleration drops to 0 (or negative, when

air resistance is considered). Springs, compressed air, and hand-powered compression all have enough

force to shoot the dart:

6

𝑣𝑑𝑎𝑟𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑙=

𝑥𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑𝑏𝑦𝑏𝑢𝑙𝑙𝑒𝑡

√2 × ℎ𝑓𝑖𝑟𝑒𝑑

𝑔𝑒𝑎𝑟𝑡ℎ

𝑣𝑑𝑎𝑟𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑙=

21.336𝑚 (70 𝑓𝑒𝑒𝑡)

√2 × 1.219𝑚 (4 𝑓𝑒𝑒𝑡)

9.81𝑚𝑠2

= 42.8𝑚

𝑠

∆𝐾𝐸 = 𝐾𝐸𝑓𝑖𝑛𝑎𝑙 =𝑚𝑑𝑎𝑟𝑡 × 𝑣𝑑𝑎𝑟𝑡𝑖𝑛𝑖𝑡𝑖𝑎𝑙

2

2= 𝑊𝑓𝑖𝑟𝑖𝑛𝑔 = 𝐹𝑒𝑥𝑒𝑟𝑡𝑒𝑑 × 𝑑𝑏𝑎𝑟𝑟𝑒𝑙

. 0015𝑘𝑔 × (42.8𝑚𝑠

)2

2= .508𝑚 (20 𝑖𝑛𝑐ℎ𝑒𝑠) × 𝐹𝑒𝑥𝑒𝑟𝑡𝑒𝑑

𝐹𝑒𝑥𝑒𝑟𝑡𝑒𝑑 = 2.7𝑁

𝐹𝑠𝑝𝑟𝑖𝑛𝑔_𝑚𝑎𝑥 = 133𝑁 (29.9 𝑙𝑏𝑓) (𝑠𝑒𝑒 𝐷𝑒𝑡𝑒𝑟𝑚𝑖𝑛𝑖𝑛𝑔 𝑡ℎ𝑒 𝑆𝑝𝑟𝑖𝑛𝑔)

However, some mechanisms are better than others at imparting more force to the dart. Mechanisms

which deliver air to the dart (pressurize the barrel behind the dart) faster can give the dart more force

and acceleration before the dart exits the barrel (fig. 5).

Figure 5. Force exerted on dart vs time plot (Impulse).

The green line represents a gun where the pressure behind the dart is increasing as the dart is accelerated,

the blue, a gun with constant pressure on the dart during acceleration, and red, a gun with decreased

pressure on the dart during acceleration. The black line on the bottom of the graph is the force of kinetic

friction acting on the dart. The shaded areas under each curve are the net impulses imparted on a dart under

these three conditions. The graphs of work (Force vs distance traveled by dart) would generally appear similar

to these curves, as the distance is a product of time.

Time

fk

fs

Forc

e

7

As such, hand-powered guns, like the HAMP mentioned above, are generally inferior in range to an

equivalent (same canister volume and same barrel length and diameter) spring-loaded or compressed-

air gun due to a slower air compression.

Dead Space and Air Leakage Dead space, the space of air that remains between the plunger’s head after the gun is completely fired

and the back of the dart before firing, is detrimental to the firing of the gun. This space only serves to

add volume to the air canister, reducing the rate of pressurization in the canister and lowering the

maximum possible achievable firing pressure. Air leakage also is a detriment to firing power. Leaking in

the canister-plunger connection, canister-barrel connection or the barrel-dart connection all decrease

the gun’s efficiency as some energy, in the form of pressurized air, is lost as the air escapes and is not

used in accelerating the dart.

Creating a Gun Design

Determining the Gun Body Lengths The lengths of all the pieces in the gun were determined more by what I could physically handle. Most

dimensions of the gun were based off of what I could pull back comfortably, 9 inches. This length

determined the size of the air canister, plunger, spring, and gun body. The plunger and canister would

need to go 9 inches forward and to fire the gun, the plunger and canister needed to be the same length.

Additionally, the spring needed to be able to be compressed 9 inches so a spring longer than the 9

inches needed to be selected (as compressed springs still have a certain minimum height and can only

be compressed so far).

Determining the Materials Needed To find out which pieces would be needed for the gun, I first created the basic design, saying that tube

would need to be able to either slide or be glued inside larger tubes and not worrying about tube sizes.

To finalize the design, I went to see what pieces were properly sized and assigned material types based

on each pipe’s ability to fit inside another pipe. Loose gaps for immovable pieces could be bridged with

electrical tape and glue or pins if the joints needed mechanical support. Sliding pieces needed to fit into

each other well, making it the obvious choice to use the 1” PVC and the ¾” couplers as the main sliding

components and the 1 ¼” tubing as the main body of the gun.

Pipe/Rod Name Fits Inside How Well It Fits Inside Additional Notes

¼“ Steel Rod ¼“ PEX Somewhat Tightly

¼“ PEX ½” Schedule 40 PVC

VERY loosely Used in the handles.

3/8“ Steel Rod ½ “ PEX and CPVC Somewhat Tightly

½“ Wood Dowel ½” Pipes Loosely

½“ PEX and CPVC ¾” Pipes VERY loosely PEX used in pieces touching the 3/8” steel rod. CPVC used in barrels. They have slightly different inner diameters.

8

½” Schedule 40 PVC ½”coupler Perfectly

¾” PEX 1” Schedule 40 PVC VERY loosely

¾” Schedule 40 PVC ¾” coupler Perfectly

¾” Coupler 1 ¼” Schedule 40 PVC

VERY tightly

1” Schedule 40 PVC 1 ¼” Schedule 40 PVC

Tightly Fit is slightly more lax than the ¾” coupler

1 ¼” Schedule 40 PVC

1 ¼” coupler Perfectly

1 ¼” Coupler 2” PVC Loosely

Glues and other fasteners were very important in the design of this gun. For anything not involving

plastic and needing a filler material, I chose to use gorilla glue, an expanding polyurethane glue. For

bonding taped connections to plastic, I used Weldbond glue, a very strong glue-all adhesive. For any

tightfitting PVC to PVC connections, I chose to use PVC cement, as the cement is designed for these

connections. For any unconventional, gap-filled, or partial PVC to anything connections, I used a plastic

epoxy called Plastaid. This epoxy bonds exceptionally well to plastics and forms solid (but brittle

connections) which are unfortunately weaker than a properly made PVC cement joint and cannot be

used with oily surfaces. Lastly, for connections not involving any PVC, I used epoxy, which glued the

pieces together very strongly.

Creating the Air Canister The canister and plunger are two pieces meant to work together to provide the pressurized air in the

gun. The canister needed to be made of 1 inch PVC, have a coupler at one end which connected to the

barrels, and have handles on its side. The handles were by far the hardest and most delicate part of this

process, as they needed to be able to support a large load without snapping, yet could not, in any way,

bore into the canister as this would create leakage and roughness on the inside of the canister which

would lead to the destruction of the plunger’s seal. The canister’s final design was 1” PVC tubing, with

an epoxied ½” coupler on the front, with two handles, metal rods attached to extended PVC bases (fig.

Figure 6. Adhesives.

Adhesives used in the creation of this gun. From left to right: Gorilla

glue, PVC cement, Weldbond, epoxy, Plastaid.

9

7), attached via plastic epoxy and small nails to the side of the canister body. The plunger needed to fit

inside the canister so it needed to be made of ½” PVC.

Creating the Plunger The plunger was a length of ½” PVC with a ½” to ¾” coupler at one end acting as a catch face for the

trigger. The body of the PVC pipe was bored full of holes to reduce its weight so that it would be

accelerated faster by the springs behind it. The seal to the barrel was one of the most difficult parts of

the gun’s creation. Many materials were tried to make a seal, cotton threads, Paper cones, foam

wrapped around the front of the plunger, smooth packaging and electrical tapes, caulk, hardened

polyurethane glues/fillers, hybrid designs including tapes, papers, and bevels on the surface of the

plunger, and carved foam. The only designs that produced a seal strong enough to fire a test dart out of

the front of the canister (which I was unable to do with lung power) were the wrapped and carved foam

designs, the carved foam design being the best. However, all these designs seemed insufficient in

creating a good seal and prone to breaking down easily (particularly wrapped foam, tape, caulk, paper,

and cotton threads, which peeled off or unraveled after a few uses). In the end, I decided to use a U-cup

airtight seal, bought commercially, which would assure a perfect seal inside the air canister. Testing this

seal, I found myself unable to compress the plunger into the air canister once I blocked off the open end

of the canister, indicating a perfect seal. With lubrication, this seal has less friction than my previous

seal I used on my earlier guns and has less friction and mass too, all making it a superior design (fig 8).

Talking to a neighbor who saw my project and had built guns in his childhood, I was recommended wet

newspaper to act as a seal, although I doubt the integrity of such a thing and its requirement of water

would make it an impractical design.

Figure 7. Attaching the handle

One of the hardest parts of this project, the handle here is being glued with Plastiad. The PVC

base on the handle is to increase contact area with the canister. Paper scraps are used to

prevent Plastaid from dripping into the gun and permanently locking the canister in place. This

particular design is the first design I had, with no nails. It broke off on the first pull backwards.

10

Determining the Spring The spring was one of the specialty pieces due to its unconventionally large length. I did not want to

have to custom order a spring, as this is very expensive, and instead ordered from McMaster-Carr,

which stocks a wide variety of springs. According to Nerfhaven:

“Constant of x inches of spring = [McMaster constant] / [Coils Per Inch * x] “

However, it seems the site has been re-arranged since then and the catalog is different. Additionally,

the coils per length is not listed on the site anymore, complicating things. However, there are

schematics of each spring and since a scale is provided indirectly through the schematic’s measurement

line of the spring diameter, the coils per inch and thus the spring constant could be calculated with a

simple proportion. Using this knowledge, I selected the 9637K31 and 9637K32 springs to go into the gun

to couple with an older spring (Ar-15 spring) I had bought for an earlier project.

The springs bought were based on my own strength, using a simple test apparatus. To test my ability to

compress via chest muscles/pectorals (the movement I guessed was required to prime the gun (which

was wrong)), I used surgical tubing to wrap around two posts, holding one end with my hand and the

other with a spring scale in my other hand. I pulled until I was unable to pull anymore comfortably and

looked at the force being measured by the scale. My estimated strength was around 20 -30 lbs so I tried

to find springs that would match this.

The Ar-15 spring has a spring constant of 1.47 lb/in (fig 9), and, when multiplied by 9 inches (the

pullback for the gun), came out to be 13.23lb , leaving up to 16.77lbs for the other spring. The spring I

selected, the 9637K32, had a spring constant of 1.9lb/in and would be compressed 8 inches (it was

shorter than the Ar-15 spring and would fit inside the Ar-15 spring). Cumulatively, the force needed to

cock the gun would be 29.9lbs.

𝐹 = 𝑘𝑥 = 8𝑖𝑛 × 1.9𝑙𝑏

𝑖𝑛+ 10𝑖𝑛 × 1.47

𝑙𝑏

𝑖𝑛= 29.9𝑙𝑏

Figure 8. Comparison of plunger heads.

The piece on the left is the U-cup seal used in this design. It perfectly seals and has relatively low

friction. The piece on the right is a plunger-head made for a previous project. It has far more

friction and an imperfect seal.

11

Determining the Barrel Length The length of the barrel, optimally, is the point where the moving dart experiences an equal amount of

air-pressure force and kinetic frictional force. In other words, the point where the barrel should end is

where the work done by the air behind the dart is equal to the work done by the barrel’s friction against

the dart. (picture here). Setting up a formula to calculate for this length:

𝐹 = 𝑃 × 𝐴

𝑃𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟+𝑏𝑎𝑟𝑟𝑒𝑙 × 𝑉𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟+𝑏𝑎𝑟𝑟𝑒𝑙 = 𝐾

𝑉𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟+𝑏𝑎𝑟𝑟𝑒𝑙 = 𝑉𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟+𝑏𝑎𝑟𝑟𝑒𝑙𝑖𝑛𝑖𝑡𝑖𝑎𝑙+ ∫ 𝜋 × (𝑟𝑏𝑎𝑟𝑟𝑒𝑙)2 × 𝑥𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑𝑏𝑦𝑏𝑢𝑙𝑙𝑒𝑡 𝑑𝑥

𝑙𝑏𝑎𝑟𝑟𝑒𝑙

0

− ∫ 𝜋 × (𝑟𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟)2 × 𝑥𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑𝑏𝑦𝑝𝑙𝑢𝑛𝑔𝑒𝑟 𝑑𝑥

𝑙𝑏𝑎𝑟𝑟𝑒𝑙

0

Figure 9. Spring constant testing apparatus

A spring suspended from a stationary clamp attached by a twined plastic bag. The uncompressed length of

the spring is marked on the side of the clamp. The scale is pulled down until the spring elongates one inch.

The scale’s reading is the spring constant in lbf/in. Pulling the spring instead of pushing it can be used

accurately to measure spring constants as steel has very similar material properties in tension and

compression pre-deformation.

12

𝜋 × (𝑟𝑏𝑎𝑟𝑟𝑒𝑙)2 × 𝑥𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑𝑏𝑦𝑏𝑢𝑙𝑙𝑒𝑡 =𝑃𝑐𝑢𝑟𝑟𝑒𝑛𝑡

𝑃𝑖𝑛𝑖𝑡𝑖𝑎𝑙× 𝜋 × (𝑟𝑐𝑎𝑛𝑖𝑠𝑡𝑒𝑟)2 × 𝑥𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑𝑏𝑦𝑝𝑙𝑢𝑛𝑔𝑒𝑟

Solving for a set of barrel lengths, this formula would have been able to produce a graph of the force

being exerted on the dart by air pressure. Subtracting kinetic friction in the barrel, which is constant and

can be easily measured by pulling a strung-up dart through a barrel by a scale, the point of intersection

between the two curves would mark out where to end the barrel.

Unfortunately, this formula is circular. The distances traveled by the dart and the plunger are both

dependent on the internal pressure, which is what I was trying to find in the first place. There are

multiple ways to find this distance, the second, longer way (not shown) using kinematics and Newton’s

second law also relies on a knowledge of the pressure in the barrel, which, itself, is a function of both

the plunger and dart position per a given time or position in the barrel. I cannot omit the existence of

pressurization (by assuming incompressible flow) in the barrel and canister as its role in firing of the gun

must exist (to pressurize the canister before the dart begins to move) nor omit the change in volume

that occurs in the barrel-canister system as the dart travels down the length of the barrel. In the end, I

need either a device to measure the volume in the canister at a given time, or easier, just get a pressure

sensor inside the gun. This formula is too complicated to calculate with my current knowledge of

physics and calculus.

At this point, I knew:

The gun barrel generally needs to be longer than a few inches to properly accelerate the dart

The volume in the barrel and the dead space needed to be less than the canister’s volume as at this

critical volume, friction was certain to be doing more work against the dart than the air was doing for

the dart.

I happened to find a formula for barrel length by a Ph.D student for fluid dynamics and attempted it:

𝐿𝑏 =𝜋 × . 5345𝑖𝑛2 × 9𝑖𝑛 × (

14.7𝑝𝑠𝑖14.7𝑝𝑠𝑖 + 20𝑝𝑠𝑖 (𝑐𝑜𝑛𝑠𝑒𝑟𝑣𝑎𝑡𝑖𝑣𝑒 𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒)

)1/1.4 − 0

𝜋 × . 1875𝑖𝑛2= 13.89𝑖𝑛

13

My original calculation of this formula was skewed as I had incorrectly measured the barrel diameters

with calipers, producing a length of 35 inches. This length I found to be annoying to have to cut and

instead settled for 20 inch barrels, which would weigh less, have most of the benefit of the supposedly

optimal barrel (as most of the acceleration happens the farther back the dart is in the barrel), and could

be cut nicely from 2 10’ lengths of pipe.

Determining the Dart Storage System One requirement of the gun was that it would be able to fire multiple times without having to be

reloaded. By this requirement, the dart storage system needed to be more than a simple barrel-break

or slide-lock mechanism. Many mechanisms currently exist which can hold more than one dart at a

time. The most common are revolver-type barrels, which have numerous barrels, each one holding a

dart, magazines, which stack darts vertically and push them into the gun by spring power, hopper

systems, ugly (in my opinion) storage systems which store darts front to back in a long tube, using

gravity to pull them into the chamber. The magazine is the most efficient design to hold the darts,

having by far the greatest dart capacity out of any of the designs for its size. However, from my past

experiences with magazines, creating the opening in the gun breach to load the dart, creating a solid

magazine, and placing an appropriate spring on the inside of the magazine are all difficult tasks and with

my skills and tools, still unattainable. Revolver-type barrels can hold less than 20 darts (more becomes

too cumbersome) and are, for me, easier to construct than magazines. Hopper systems are the easiest

way to store multiple darts, but have numerous flaws. They require the hopper to be pointed

downward, are cumbersome to carry around due to the hopper’s length, have a bad air-seal due to the

barrel needing to be significantly larger than the dart for the dart to be properly loaded, and lowered

firing efficiency, due to the hopper acting as dead space when the gun is fired. Some of these flaws,

particularly the larger barrel and the lowered firing efficiency are reduced when this system is used with

the more complicated vacuum-loaded guns. However, the hopper system’s overall impracticalness and

numerous issues and my confidence in creating a dart-drum, the collection of barrels making up the

front of the gun, persuaded me to choose a revolver-type system for the gun.

The issue I have had with revolver-type systems in the past is the indexing system, the system that turns

the drum 1 barrel over each time a dart is fired. The particular cam needed for this design is shown

below (fig 10). Typically, this design is very difficult to create accurately, especially without the use of

precision tools. However, I realized that my design could be made possible if I, instead of cutting

multiple grooves into plastic, cut a groove once and had multiple “treaders”, rods which followed the

cam, on the drum instead. Had I not found this solution, I would have tried a different approach due to

the near impossibility of cutting perfect grooves into pipe with just hand tools.

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Determining the Trigger The trigger is one of the hardest parts of the gun to construct, owing to the fact that it often requires

multiple moving pieces, must translate motion from one location on the gun to another, must change

the nature of the motion reliably, and must be able to resist the strong push of a spring inside a cocked

gun. For me, the trigger needed to be simple and scarce of many moving parts due to my limited tools

and the tendency for more complicated mechanisms to fail more often than simpler ones. Originally, I

had planned for the trigger to be a SNAP trigger which was quite easy to make. However, I didn’t like

the design too much, particularly the feeling that you would get pulling the trigger and the fact that the

trigger didn’t look very sturdy. When the gun was designed, I realized the trigger would need to be

behind the handle/grip and the SNAP trigger, which can only be used at the handle of a gun, wouldn’t

work. I looked into other triggers to use and decided to try the purple catch, although this seemed

complicated with my machining ability and prone to failure and unreliability. In the end, I settled for a

design of my own making. In my design, the trigger acts as a lever/spool and grabs around the wire.

The wire is connected to a screw attached to the end of the one way catch, a piece made of two parallel

cuts into PVC pipe, a Plastaid welded, triangular catch piece, and a screw. Upon loading the gun, the

plunger is pulled over the triangular plastic in the catch, being allowed to slide backwards, but unable to

slide forwards again due to the catch face rising up after the plunger has passed and buffeting the

plunger’s flanges. When the trigger is pulled, it rotates, creating tension in the wire. The wire pulls on

the screw in the catch, creating a torque. This torque pulls down the catch face and allows the trapped

plunger to rocket forward and fire the gun. No complicated shapes needed to be cut out of plastic (or

3d printed) and this catch can be placed anywhere on the gun, should the need arise in another model,

with little complication.

Figure 10. Barrel cam system for intermittent rotational movement from reciprocating linear

motion. (Taken from Mechanisms and Mechanical Devices Sourcebook 3rd edition)

The cam needed to translate the linear motion of the gun priming to the necessary rotational movement.

Alternative solutions are very similar to this particular one. To reduce the complexity of the cam, my

design uses, instead of multiple grooves and one treader/indexing pin, a single groove and multiple

indexing pins. For my design, see figure 2. For an animation of this mechanism, see here.

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Problems Encountered While Creating the Gun and their Solutions Red=Hardest to solve/if left unchecked, will ruin the gun mechanically or performance-wise

Yellow=Moderately hard to solve/may or may not ruin gun

Green=minor problem/hack/recommendation/easy to solve/generally will not ruin gun

Problem Description Solution

GENERAL

Widening a hole You want to make a hole larger

Use a sanding attachment for a rotary tool to do this. If you’re like me and don’t have one, use the largest spiral drill bit you own, clamp the drill (and trigger) inside a vice, and place the spinning bit inside the hole, spinning the edges of the hole around the drill bit.

Plastic scraps from cuts and drills are stuck to the inside of the gun

Before any pieces are put into the gun, there are tiny PVC shavings inside the gun from your cuts.

Take a cup of water, and drop the water down the length of the gun. The water washes away the shavings quickly and leaves the gun clean.

Glue will not stick Glue does not properly bond two surfaces together.

Make sure the surfaces are clean. Unseen oils weaken any glue connections. Most glues need a just-right tightness to work, not too tight the glue is pushed off and not too loose that it leaves a large gap to be filled by the glue

STOCK

Figure 11. The trigger mechanism

The trigger unprimed (left) and primed (right). The catchpiece wedge for the trigger is located inside

the gun body. When the plunger passes over the catch backwards during priming, the catch deflects

downward but when the plunger is pushed forward, the catch stops the forward motion of the

plunger. To release the catch, the trigger is pulled, tensing the wire and pulling down the catch. The

plunger is released and the gun fires. For the other trigger designs considered, see Figure 2.

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Stock is misaligned The pieces in the stock are crooked.

Glue the bottom half of the stock first. Glue all pieces while pressing them on a flat, level surface to ensure they are on the same plane with each other. Glue the top of the stock on last, using a length of pipe on the top to mimic the body of the gun to ensure that the stock is glued on correctly.

The front of the stock is not connected

The front part of the stock, the cut T joint, will not seal to the body of the gun

Sand down the ridge on the T joint until it becomes a level surface. Glue.

SPRING(S)

Multiple springs overlap with each other

Using more than one spring, one inside the other, causes rattling and in the worst cases, the full compressed length of the springs to be longer than expected.

The easiest way to avoid all this is to use one spring. However, if you choose two springs, make sure that one spring is significantly smaller in diameter than the larger diameter spring. From personal experience, if the springs are too close in size, the coils of the small spring may be caught by the larger spring and cause the gun to be unable to be primed due to an increased spring compressed length. This was the case with the 9637K32 spring.

Springs buckle inside the gun

Pulling back the spring buckles the spring rather than compress it.

The spring must be supported in some way. This can be done by a tight external support surrounding the spring, or, in my case, the use of a wooden dowel to keep the spring compressing.

Springs are grabbed by the plunger

The spring is caught by the plunger as the plunger is shot forward

Glue the springs to the back of the gun, so that they will always be located in one place. Patch up any grooves inside or on the back of the plunger, the spring end is getting caught on a groove in the plunger.

Spring cannot be held by back of gun

When the spring is fully compressed, the back of the gun flies off (hoisted by your own petard)

Use mechanical connections. Drill into the spring endcap and gun body and insert nails through the holes

PLUNGER

The plunger is too heavy

Not a very big problem but does subtly reduce performance

Lighten the plunger by sanding off any rough edges and boring holes into the PVC pipe. Be careful to maintain structural integrity of the pipe.

The plunger seal is bad.

The seal between the plunger head and the canister is leaking, creating terrible gun performance.

Creating a plunger seal by hand is extremely difficult. The best one I made by hand was out of insulating foam, which I spun into a circle and bored out the middle. For best results, use an industrial seal (O-ring, U-cup) for a perfect seal.

The plunger seal is difficult to push into the canister

The plunger cannot easily slide inside the canister, heavily reducing efficiency.

Add SILICONE lubricant to the inside of the plunger. Sliding will become much easier. Using WD-40 or Vaseline will eat away at the rubber seal and render it useless. Do not use them.

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Handles were glued on the canister and the plunger was not inside.

The canister is permanently inside the gun body, making inserting the plunger difficult

Insert the plunger until it is pushing on the open end of the canister. Have someone else hold the canister in place while you push downward, rotating the plunger. Your helper must use his/her thumb to passively push the rotating plunger head into the canister

CANISTER

The canister-barrel connection is stopping the plunger

The front connection of the canister to the barrel stops the plunger’s momentum.

The front part of the canister is relatively weak compared to the rest of the canister. The first time I fired the gun, it shot the front of the canister off. Move the canister forward enough to leave little dead space, but enough so that the plunger is stopped by the back end of the canister

The canister’s handles are too weak

The handles, placed on the sides of the canister break off easily.

Currently, the handles are attached, somewhat firmly, through the use of plastic-epoxy and two small nails, too short to pierce the inside of the canister. In future designs, I may want to have the handles placed in the back of the canister so I can attach a coupler to give the steel handles a better grip on the canister.

The Plastaid plastic-epoxy gets into the body of the gun.

Undried Plastiad falls into the space between the canister and the gun body, causing the canister to be unmovable when dried

Use paper scraps around the canister and underneath the gun body when gluing the handle on. If you get some inside the gun, slide the canister back and try to remove it by hand. If it is stuck to the canister, move the canister back and forth until the Plastaid dries on itself, balls, and exits from the sides of the gun.

The handles can’t be pulled much in any direction but front and back

Pulling up or down cracks the handle off if enough force is applied

Currently no solution. Avoid letting smaller children try the gun, as their decreased body size causes them to pull the handle in the wrong direction (at an angle). In the future, the handle will have a larger contact area or be connected to a solid coupler

The firing sequence breaks the handles

The handles, which stop the plunger’s momentum Experience some breakage when the gun is fired

Put small bits of foam on the ends of the handles to smooth the energy transfer from the handles to the gun body. The breakage has never occurred between the canister and the handle for me, although it has broken off some external layers of epoxy on the handle

BARRELS/DART STORAGE SYSTEM

The seal between the canister and barrels is bad

There is a leak between the compressed air and the dart, causing a large decrease in performance

Directly pressed surfaces are not very good seals at this level of pressure and volume. Rotating barrel locks and my own lock, electrical tape wedged into a coupler, both have higher (in my case, perfect) sealing capacity.

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Barrels are not straight

Crooked barrels lead to inaccurate shots

First, sand the insides of all the holes and grooves you wish to store the barrels in. Second, glue two barrels on opposite sides together, clamping them so they are parallel. Glue the rest of the barrels once the endpieces of the barrels are aligned, holding them in place with a rubber band.

Barrels weight too much

Barrels are too much of a burden to be used in a dart fight.

Barrels can be made out of thin-walled tubes. In the future, the barrels will be cut very short and feed into a single, long barrel.

Indexing holes not properly placed

The drilled holes for the index pins are crooked and/or misaligned. Gun won’t be able to change barrels or be loaded more than once.

Start with a perfectly circular piece of material for the wheel. Use a drill press!

Wheel is uneven My broken jigsaw cuts with a huge slant into the wood and makes jagged edges. Makes drilling perpendicular holes impossible. Ugly.

Grip the roughest rasp you have in the vice. Take the uneven wheel and rasp it over the rasp by hand until it becomes even. Here’s where an automatic sander would come in handy.

GUN BODY

I need to cut a straight groove into the side of the gun’s body.

A groove is needed so that the handles can attach to the canister and be used to pull back the plunger

Mark where the cuts need to be made. Plunge cut the pipe with the vibrating cutter. The cutter cuts largely straight and slowly so you can correct any mistake quickly.

TRIGGER AND HANDLE

Trigger “holder” cannot be easily glued

The twin plexiglass pieces which hold the trigger are difficult to glue correctly.

Cut the grooves into the handle using the vibrating tool and make them the correct angle by using the rotary tool with a spiral bit. Insert plexiglass pieces. Place trigger piece and axle in their appropriate spots. Apply epoxy. Place ¾” coupler on top of the plexiglass pieces to keep them level. Remove this coupler before the trigger holder dries.

Trigger pull does nothing

The trigger wire is not taut enough.

Make a simple knot and slide it along the length of the wire to obtain the location of the knot needed.

Trigger keeps slipping

The wire slips off its metal pin.

Epoxy is useless here. Get a metal crimping pin here or tie a second knot around the first to secure it in place near permanently.

The metal pin connected to the catch hits the stock.

The pin is too long Cut the stock a little near the pin, or easier, cut the head off the pin.

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The catch is difficult to place inside the gun.

The catch is a small piece needing to be placed far into the gun. It requires pressure and proper placement to glue properly.

Cut open the body of the gun next to the catch and insert the pieces to be glued from the hole. This hole does not influence the gun’s performance.

The catch doesn’t catch.

The catch isn’t rising high enough to catch the plunger.

Make a taller catch. Sometimes, the issue can be the catch is too long and the canister end is depressing the catch enough to let the plunger slide over it. If this is the case, shave off the end of the catch to make it shorter.

The catch wears away

Constant catching wears away the catch and the plunger catchface.

Metal plate the catchfaces of both the trigger and the plunger.

Handle won’t glue to gun body

The handle has a weak connection to the gun body

See the solution above under issues with the Stock.

Performance

Accuracy Although accuracy is an important part of the gun, it was not tested for here due to a difficulty in aiming

the gun consistently with a detachable barrel and the fact that it is mainly controlled by the dart being

fired. However, notably, this gun fires very accurately for the distance the dart travels, with darts

staying generally within one foot and a half of the line of fire, far more accurate to NERF elite darts, the

NERF darts which go the farthest currently.

Range The gun was used to test fire standard, homemade darts, each weighing between 1.3-1.7 grams (fig. 12)

11 times. The barrel was placed at 4 feet above the ground on a table for support. The barrel angle was

practically parallel to the ground (flat fire) during each fire.

Test Fire Number Distance Traveled by Dart (feet)

1 82

2 78

3 86.5

4 90

5 99

6 80

7 96

8 84

9 85

10 102

11 89

Average: 88.32 feet

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Data Range: 24 feet

The high variance in range, I suspect, is due mainly to barrel differences (some noticeably felt harder to

push darts into to load, indicating greater friction inside them) and how strong the seal to the barrel was

(which was not constant as I was holding each barrel to the gun by hand very crudely).

Darts

Dart Physics Although the main focus of the documentation is on the gun, darts play an equally important role to the

gun’s performance. Properly made darts can increase ranges by many feet and fire perfectly straight.

Improperly made darts will not fly at all and will fishtail out of the gun almost instantly. For stability,

each dart must be made so that its center of mass, the point around which the dart rotates, is in front of

the aerodynamic center of the dart, the greater the distance, the more stable the dart in flight. Having

the dart designed this way allows the dart to self-correct its orientation in the air as it flies. To create

stability, fins can be added to the back of the dart, increasing the surface area on the back of the dart

creating lift, moving the aerodynamic center back, or, more easily, by moving the center of mass

forward by adding weight to the front of the dart. I use a small screw as my weight and it seems to fly

quite well. The weight of the screw is so much more than weight of the foam that it is difficult to lay

down the dart sideways as the screw keeps trying to push the dart into the ground face-first, a good

sign. The weight added to the front affects performance drastically. As stated earlier in Design Choice

(p3), the greater the mass of the dart, the less distance it can travel. However, there must be a balance

as too light of a dart will actually travel less distance due to it twisting around in the air more and losing

its forward momentum. My darts, to avoid issues with bowed darts spiraling out of control, are very

short to have little curvature (and are thus, more accurate), yet have a moderately heavy mass (screw)

at the front of the dart which brings the center of mass right to the tip of the dart (as the foam for the

smaller darts is very light and often cannot even be measured by my scale which has 0.1g precision).

They have completely flat heads, making them less aerodynamic but far easier to make than darts with

curved, aerodynamic noses.

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Alternative Darts Before I created my gun, I underestimated its range, thinking it would shoot less than 70 feet, and

figured I would have to make up for it by experimenting with dart designs. Aerodynamic drag is a factor

of how far a dart will travel and possibly plays a large part in the final distance traveled by the dart

(source). Originally, to reduce aerodynamic drag, I planned to use Nichrome wire (which has high

resistance and can heat up enough to cut foam) to cut noses for the darts, in approximate parabolic

shapes, by turning the darts around a parabolic Nichrome cross-section like on a lathe, cutting a uniform

shape around the top of the dart. Another idea I entertained was the idea of turning the dart into a

glider by having crushable wings on the dart which would fit around the dart during firing. I didn’t go

with this idea for long, due to the difficulty of making such a thing, the poor seal it would create, and the

fact that a glider-dart would swerve upward from being fired so fast and completely miss any target I

aimed for.

Darts for the Future: I believe that aerodynamic noses will improve gun range significantly, but have no easy way to be mass-

produced. For that reason, and the already acceptable performance of my flat dart slugs, my darts will

all have flat heads. The darts I have are currently a little too heavy for my taste, as in my tests, adding

even 0.1g to the dart can change the range by feet. To reduce weight, the dart of the future will need to

be longer than the darts currently so that the aerodynamic center can be moved backwards, while the

center of mass stays largely the same in the front of the dart. The mass in the front of the dart could be

lessened (using metal BBs (5 grain) as weights) and the dart would still be able to retain stability in the

air. Lastly, to make the darts safe for shooting people, each dart would need to have a spongy padding

on the front of the dart to dampen the impact. Currently, the darts with hard, metal heads can shoot

through 5 paper sheets pressed together and hit with enough force to cut open skin in small cuts. I

Figure 12. Darts

On the left are the two types of darts I use, a wrapped and unwrapped dart (unwrapped dart on far left).

Both darts have a screw in the front as a weight and fly about the same distance. While the wrapped

dart weighs more and has a more rearward center of mass, I suspect the similar range comes from a

greater acceleration during fire due to decreased friction.

Dart length is about 1 ¼”. Dart mass is around 1.5g for a wrapped dart and .9g for an unwrapped dart.

Wrapped darts were used for range testing.

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hope a covering of felt or foam and the lessening of the dart’s mass will be enough to lower the impact

to be enough to be safe for human targets.

Thoughts on the Current Gun I am quite satisfied with the gun currently. The gun exceeded my expectations in range, shooting past

my objective of 70 feet, and is quite accurate. Its effective range, the range at which an object can be

consistently hit is currently unknown due to the barrels not being attached to the gun but I estimate it to

be no more than 15 less than the current range of the gun. The gun’s barrels add too much weight to

the front of the gun to allow the gun to be reasonably carried and I plan to remake them so they weigh

less. Without the barrels, the gun is slightly back-heavy so that with the addition of the lighter barrels in

the future, it will become close to being balanced. The pullback to the gun is surprisingly easy,

considering how hard it was to do before ordering the springs. I feel it is a good pullback, not too strong

that it is difficult, but not too light that the spring is too weak. However, I feel I could definitely handle a

gun with a bigger spring and if I were desperate for performance, I would look into finding a stronger

spring than the two I currently have in the gun. The air seals are very good on this gun. The plunger-

canister seal is perfect; the canister-barrel seal is perfect if pressure is applied; the barrel-dart seal is

very good, leaving practically no room for the air to escape around the dart. The trigger mechanism

works very well and has a good responsive feeling. However, I worry that continued use of the trigger

will wear down both the catch face and the plunger and leave the gun inoperative. The handles of the

gun feel a little unsteady, like they will break with each pull. One handle broke off when my brother

tried to pull it back and pulled the handle down instead of back, a weakness in the handle design.

Improvements for the Future Although my next project will likely not be this gun’s successor, as I am happy with its range currently,

there are many improvements I would like to see on this gun. The first improvement would be to use a

single spring instead of two here. Having two caused some problems with compression and makes a

loud noise when fired. This spring might be a little stronger than the current one, as this one is current

quite easy to pull back (not necessarily a bad thing) and the range could be improved with a more

powerful spring. The catch mechanism could be improved too. I originally had a metal plate on the

catch-face to prevent the face from wearing away, but after a few fires, it fell off even after being

epoxied to the face’s surface. A more permanent metal catch face and plunger catch are needed so that

the gun remains operational longer. I would also need to improve the handles, as I don’t feel confident

pulling them back that they will withstand the force of the continual firing and pulling back for long. This

improvement would likely come from an increased area the handle contacts the canister or having the

handle attach to a coupler at an end of the canister for a much more solid connection. Lastly, I might

want to consider using a magazine instead of a revolver-type dart holding system. It is lighter and can

hold more darts easier and I feel that maybe my skills have increased enough to chance it again.

However, in the near future, I plan to stick to the revolver method and cut the barrels down to reduce

their weight to be able to use it on the gun practically. Mentioned above, I also plan to modify the darts

so that they are safe, lighter and have a greater range than the current darts. For more minor details, I

plan to paint the gun so the measurement marks on its surface are hidden, add an iron sight for use in

aiming the gun (the gun is accurate enough to warrant a sight), add a light for use in the dark, and pad

the handle and stock with Oogru, a homemade, silicone, moldable substance which can make strong

and squishable surfaces for comfort (although the gun is quite comfortable now).

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In terms of materials, I would definitely invest in a drill press attachment for my drill, as angled bores

required far more time to correct than I thought and drilling into curved surfaces was very difficult. To

make things easier, I would also want to get an automatic sander, as the rasps sometimes were too slow

and could not reach smaller parts of the gun (where I had to resort to sandpaper).

My design process could also stand to have some improvement. This gun was created from a set of

rough sketches (fig 2), with many details missing, including exact lengths and details on the barrel

(material, number of barrels). Next time I create a design, it would be more time-efficient to design the

gun with more detail so that time would be saved during construction and potential permanent damage

to the gun would be avoided. In the future, possibly the use of CAD software would also be very helpful

in the design process. This design was relatively simple but I often had to pause to think about how the

pieces came together. Drawing the entire design on the computer would make the design process less

prone to unexpected problems and easier to share with any collaborators.