What does firearm caliber measure?

[David_Simmons]

@JRDelirious

Very few weapons will fire properly, or even function at all, if loaded with different-from-spec ammo, and the few that do accept only a minor variation – the most notable being a .357 Magnum revolver, which will easily fire .38 Special rounds because they are just shorter and with a lighter load than the .357 Mag, but otherwise fit the chamber exactly. With almost any other weapon, wrong ammo will either not work or damage the weapon, perhaps catastrophically.

[anectode] In the movie Sergeant York, Gary Cooper as York uses a Luger. York used a Colt .45 which was standard Army issue for non-coms. A Luger had to be used because the blanks that were used didn’t have enough kick to operate the slide on the Colt.[/anectote]