Sherman Tank Myths - The 76-mm M-1 was the 3-inch M-7 Barrel Fit to the 75-mm Gun Breech

According to what Ive read the entire purpose of the 76-mm M-1 was to find a weapon in the 3-inch class better suited for the M-4 tank itself, something that was not as heavy and unwieldy as the actual 3-inch M-7 gun being used on the M-10 tank destroyer. The British eventually shoe-horned their 17-pounder into the turret of the 75-mm tank, so the situation was not a matter of the 3-inch gun not fitting; it was a matter of wanting a much better fit. The Ordnance Department undertook this task on their own and had plenty of time to play with it, so there was no rush to cram a gun into a tank.

It can be difficult to find detailed explanations of the development of the 76-mm M-1 gun and what it actually is and how it fit into the 75-mm, 3-inch, 90-mm scheme.

One of the first explanations of how the 76-mm came about was that the Ordnance Department finally hit upon the idea of fitting the 3-inch barrel to the 75-mm breech ring and using a slender cartridge case. The Armys own TM 9-1901 Artillery Ammunition technical manual describes the 76-mm as: The 76-mm gun is a composite of the 3-inch gun tube, and the 75-mm tank gun breech ring and mechanism. Perhaps that is the source of said statement.

Said description, though, is wrong on two technicalities.First: the 76-mm used a far more slender and shorter cartridge case. As TM 9-1901 puts it when explaining the differences and similarities of the 3-inch and 76-mm: the completed rounds are not interchangeable, since the chamber of the 76-mm gun is smaller and requires a shorter cartridge case as well as a different weight of propelling powder.

What that means is that the barrel of the 76-mm cannot be the 3-inch gun barrel attached to the 75-mm breech ring: the barrel and chamber of said guns form a composite part called the tube which is cast and bored out as one piece. The 3-inch barrel might have been used as a basic idea (dimensions from chamber forward or such), but the 76-mm could only be developed by taking a tube blank and boring it out with a chamber for the 76-mm round.

A second flaw is explained by the description of the development of the 76-mm given in M-4 76mm Sherman-Medium Tank published as Osprey New Vanguard #73. The 76-mm T1 started with a 57-caliber long barrel. This was to front-heavy for the mounts for the M-4 tank so the length was reduced to 52 calibers. Thus, the 76-mm started out as an entirely new gun, not simply a 3-inch barrel fit to a 75-mm breech.

Not that the 3-inch might not have been used in the exploration phase and formed much of the basis. But, avoid the simplistic explanation some people might give that the 76-mm was just the 3-inch barrel with the 75-mm breech. Technically that is not possible and from all evidence the original intent was to develop a weapon with a longer barrel than the 3-inch.