The Civil War version I have heard and read was that because of the numbers of .577 Enfields and .58 Springfields in use, commanders of units armed with Enfields would, when issued .58 Springfield ammunition, have their troops load the bottom of their cartridge boxes with the more correct .577 rounds and the tops with .58 rounds, as the troops fired off the .58 rounds and their muskets became fouled they could then use the proper .577 rounds.
The Vietnam version is that "they" could use our 81MM mortar rounds in the Combloc 82MM mortar. I know of no one who has tried firing 5.56 ammo out of any firearm chamber for 7.62 x 39.
One time I fired-inadvertently-a 9MMP round out of a 40 S&W barrel in my Browning HP. It functioned normally, ejected properly. Accuracy was.....