What is the Caliber Length Designation of the US 75-mm M3 Gun?Copyright 2015 J.D. Neal

Military technologists typically express the basic characteristics of cannon by their bore diameter (such as 3-inch or 75-mm) and a caliber number. The caliber number can be expressed in multiple ways, including 75-mm L/37.5 or 37-mm/50 or even 47/50.The caliber in this case is short for caliber lengths of the barrel. Thus a 57-mm L/50 cannon has a barrel that is 57 x 50 = 2,850 millimeters long or about 112 inches. A 75-mm L/37.5 gun has a barrel length of about 2,812.5 mm or 111 inches.A layman will (in disgust) spout the frustrated words: theyre about the same length so why dont they just use inches, feet, millimeters or centimeters or meters! The caliber designation is mainly intended to help give a quick glance at a weapon. Guns tend to be around 25 calibers or more; and howitzers 15 calibers or so or less. Once you get used to it, you can tell whether a weapon has a relatively long or short barrel.

Having the same barrel length does not make them the same basic gun. The British 57-mm L/50 cannon uses a cartridge almost exactly he same size as the USA 75-mm L/37.5; British loads often have about 2.3 pounds of propellant powder versus the 2-pounds common with the US guns.The layman might then assume they have much the same velocity and power.Not true! The 6-pounder has a velocity of around 2,950 feet per second (varying by load); the USA 75-mm had a velocity of only 2,000 feet per second.As far as power goes, the 75-mm makes up somewhat by firing a 15 pound shot versus the 6-pound shot of the 57-mm. The 75-mm actually has more energy than the 57-mm. But, the 57-mm has a smaller frontal area which means they both tend to have about the same armor penetration ability if not a slight nod to the 57-mm. Anyway: there have been multiple quotes about the caliber designation of the US 75-mm M3 gun. These vary from L/40 to L/32. The author really didnt have the time to rack these downs and glommed onto the designation L/37.5. But, what should it really be? It certainly isnt L/32 and whoever is quoting that number does not know what they are talking about. Perhaps they are referring to the 75-mm M2 which has a shorter barrel; but it has a correct caliber designation of L/28.5 (or so). Perhaps someone added the correct caliber lengths and divided by 2 to average them but that should give 33 not 32.To continue: BRITISH AND AMERICAN TANKS OF WORLD WAR TWO indicates that the 75-mm M3 has a tube length of 110.625 inches (which is almost exactly 37.5 calibers long) and overall length of 118.375 inches (40 calibers.) Thus we find the source for quotes that it was a 75-mm L/40.To determine which measure should be used, look at the 76-mm L/52 M1 series gun in the same book: it has a tube length of 156-inches (52 calibers) and overall length of 168-inches (56 calibers).Thus the official caliber designation is based on tube length.Consider the British 6pdrs Mk 3 and 5 with tube lengths of 96.2 and 112.2 inches, for calibers 43 and 50, which is what is typically quoted. The 75-mm Quick Firing Ordnance has a tube length of around 112.6 inches for a caliber designation of 38.1. Which is an interesting number because it is almost exactly the same designation as the USA 75-mm L/37.5. This is not a coincidence; after using the US 75-mm gun in the M3 and M4 medium tanks in Africa, the British became so enamored of its high explosive shell (up until then the tanks and anti-tank guns they were using were the 2-pounders and 6-pounders with somewhat weak high explosive shells, if any) that they (a) used 75s from wrecked US tanks to re-arm some of their own tanks as the 75-mm NA, and created their own version based on the 6-pounder up-graded to 75-mm caliber, chambered for the same ammunition used by the US gun.