If you pick a Glock, then of course you have to decide on what caliber and cartridge type.
For naval rifles, the term "caliber" refers to the length of the barrel to the diameter of the block. So, a gun might be 16 inch 50 calibers.
WW II tank guns had the same sort of designation. The Panzer IV started out with a short 75 mm main gun, 30 caliber I think it was, and later was fitted with a more powerful 75 mm gun in 75 calibers, I probably have the second 75 wrong. I'd hafta look it up.
The Panzer IV F2 and up were upgunned to try and handle the Soviet T34 and KV tanks.