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Hey, that's pretty interesting! Thanks for sharing it

And yeah, the right gun had an "R" below. It's interesting, though: the guns are not actually original, they've been replaced by Soviet-made 100mm guns by swapping out the barrels.

I always thought the breech would have to be swapped out as well, but I guess I was wrong.

Barrels thread or otherwise lock into the actual gun; it’s quite possible to make slight chances in barrel caliber and keep using the old size of propellant charge and only a slightly modified casing. You would only run into trouble doing this is the weapon already fired a straight walled shell casing (it would mean the shell is going to be wider then the casing, thus impossible to load), but that's rare for artillery and uncommon in small arms larger then pistols.

You can find plenty of cases of gun barrels being just straight bored out to take larger ammo even. That’s why the Imperial Germans used 77mm field guns for example; it allowed captured 76.2mm and 75mm guns to be bored out to use German ammo.

The soviets also used a larger mortar than other nations - apparently for allowing it to use captured ammo, but not vice versa. Could German 77mm fire 76.2 and 75mm ammo or did this only apply to mortars?

Thanas wrote:The soviets also used a larger mortar than other nations - apparently for allowing it to use captured ammo, but not vice versa.

Yeah 82mm instead of 81mm. For a while in the early part of the Cold War it was suggested the US Army should convert to 82mm too; this was within reason because we had already fired off most of our existing 81mm stockpile in Korea and yet could not use lots of captured communist ammo we picked up in the conflict. Didn't happen because the US Army had no funding for anything that was not nuclear in the 1950s.

Could German 77mm fire 76.2 and 75mm ammo or did this only apply to mortars?

With smooth bore mortars firing bagged charge ammo you could get away with firing 81mm out of 82mm. Rifled artillery firing with metal cartridge casings is not so forgiving. It might be possible to fit a larger driving band on a 76.2mm or 75mm shell to allow it to fire from a crimped 77mm shell casing out of a 77mm gun, but it would be more trouble then it was worth. Instead what the Germans would do is fire off the captured 76.2mm or 75mm ammo until it was gone, then send the gun to a factory to be bored out to the larger caliber so it could use German 77mm ammo. This was damn useful since gun barrels are expensive and might only last 5,000 rounds or so. You could use up that lifespan in WW1 in a few months of fighting; though 77mm was first adapted back in the late 1870s IIRC when modern artillery really started taking shape.

Back in the days of blackpowder smoothbore cannons though it was very common to fire ammunition that didn't fit right, you would just wrap more layers of cloth around the cannon ball until it was snug in the barrel. Of course they would also just ram stones and anything else they had on hand down the barrel if ammo supplies ran low.