It’s technically possible though. The case of 38sp -> 357mag is a working example. The cartridge is longer and pressures higher so you don’t want to load a 357 into a 38 (and often can’t). But you can run 38sp in your 357 all day long.
One warning I received from my gunsmith though is that you’ll want to clean the barrel super well when switching so the powder ring of the longer round doesn’t cause problems. Hasn’t blown up in my face yet. Will report back if that changes.
Sure there are select cases where it is possible or practical. .38 Special/.357 Magnum as you point out, I believe there are revolvers that let you shoot .45 ACP or .45 Long Colt with a cylinder swap, as well as the same for .22LR and .22 WinMag.
However in the GP's context, the typical small arms ammunition for NATO (5.56x45mm and 7.62x51mm) and the Warsaw Pact (5.45x39mm, 7.62x39mm, 7.62x54R) are not close to being interchangeable.
I've fired .22LR out of my .22 WinMag. It's possible but the casing will sometimes split as there is more room for expansion. The particular cheap brand of .22LR I was using only did this about 10% of the time. Not a big deal for a bolt action but I could see that leading to jams in autoloaders.
I was thinking of the Ruger Single Six, where you swap cylinders to shoot the two rounds. Not sure I'd want to try it in an autoloader. The difference in cartridge length not fully engaging the bullet into the throat can't be good for it.
E: Not to mention the excess fouling in the chamber and even back into the action. Cleaning that is no fun.
> The case of 38sp -> 357mag is a working example.
That was intentional on the part of the people bringing tree-fiddy-seven to market. Backwards compatibility to something cheap and readily available is a great way to ensure more people buy guns chambered in your new and uncommon at the time caliber. They could have made it forward compatible too (i.e. chamber tree fiddy seven in your .38) but didn't because that would have been wildly unsafe.
One warning I received from my gunsmith though is that you’ll want to clean the barrel super well when switching so the powder ring of the longer round doesn’t cause problems. Hasn’t blown up in my face yet. Will report back if that changes.