Comment by ggreer

11 hours ago

I think the oldest cartridge that would still work in a modern firearm would be .22 Short, which is from 1857.[1] It was developed into .22 Long, which was developed into .22 Long Rifle, which new firearms are still chambered in today. It wouldn't cycle a semi-auto though. Another contender would be .38 Long Colt, which is from 1875 and will work in firearms chambered in .38 Special or .357 Magnum.

The main causes of incompatibility were the transition from ball-and-powder to rimfire cartridges, then the transition from rimfire to centerfire. It's surprising how little firearms have changed in the past century. 9mm Parabellum is 125 years old. The Browning short recoil action (used by most handguns today) is over a century old.