Most MCGs that don’t require modification to shoot multiple calibers typically just fire cartridges of the same bore diameter but differing power. Less powerful cartridges are often cheaper and put far less stress on the weapon (increased longevity). I list the longest cartridge first.
.22 Long Rifle (LR) / .22 Long / .22 Short: Nearly all revolvers and tube-fed, non semi-auto (bolt, level, pump) rifles that fire .22 Long Rifle will fire their older, weaker .22 Long and .22 Short cartridges just fine. Semi autos designed for the .22 LR won’t cycle these weaker cartridges but can be used as a single shot. The utility is questionable as .22 Long and .22 Short are much, much less common than .22 LR. .22 Short is fine for pest control in built-up areas but in a true grid-down SHTF scenario I think subsonic .22 LR will be much, much more useful. Also, the shorter cased .22 Long and .22 Short can build up lead in the chamber (making shooting .22 LR difficult until cleaned) and worse, with continued use can fire-cut the chamber directly in front of the case and ruin it for .22 LR shooting.
