Came out of the end of WWI. 75mm guns were found to be relatively small, yet able to make things ded. As tanks became popular, it was found they could be really good at punching through armor, without being too fuckhuge to move around. 76mm was just a slightly different size that the ruskies really liked, until it almost all of it on the eastern front got captured by the germans during opening weeks of barbarosa.
Because the diameter was 1mm different but the performance of the rounds were much different. A Sherman with the standard 75 mm was relatively undergunned, but with the refit to the end war 76 mm made it on par or superior to the other tanks fielded.
A .22 LR and a 5.56 NATO are essentially the same diameter bullet, but the cartridges and the performance of each are vastly different. The 76 mm gun had a much faster moving projectile.
>>28427476 I'm guessing you're talking about the difference between the American 75mm and 76mm guns.
Both were really the same caliber but the Ordinance Department designated the 76mm as such to prevent higher-pressure 76mm shells being given to Shermans thst still had the lower-velocity guns. Other than that, the 76mm had a higher velocity and as such, a thicker she'll casing and a larger she'll cartridge.
The way I was told why those were the sizes was because of pre-existing shells just being multiples of those diamaters. 76mm is 3 inches, 152mm is 6 inches, and so forth. This doesn't seem to hold true across the board though.
Well it's a number of reasons, op. Also, standard shell for WHAT? It's kinda misleading, I guess you mean "the average tank/field gun in WW2 was a 75mm or close to that"
Mostly it had to do with previously standardized WWI calibres being the starting test bench for ever more powerful loads, once you build a turret around a 75mm sized gun, working with other calibers can be hard. About field guns, it was a matter of having a gun with a carriage that could be moved without being a logistical nightmare.
The germans, for example, had many short howitzers in 75mm, so they had many gun carriages and AFV designs built around guns of that size. When WW2 showed that AT guns needed to be much more powerful (at the time the germans were fielding 30 or 50mm guns in the AT role), the first thing they did was taking the 75 design and reworking it so it would be a long barreled high pressure gun, then it was easy to adapt carriages and turrets to that gun. Similar ideas were tried with 105mm guns, another "standard caliber" of the time, but field carriages for such a gun were too heavy and big, while no turrets could house such a large gun reliably. Many nations tried at the same time to adapt ever bigger guns, but in the end the "next class" of high pressure AT guns ended up being 85mm (russians)/88mm (germans)/90mm (allies) all mostly based on pre-existing AA guns.
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