Comments

Accidentally hit post. .556 and .223 have different shoulder dimensions and SOME riffles can use them both while many cannot due to spacing. A 7.62 is close to a .308rem with similar caveats. The 9mm was developed primarily for smgs but took foot as a pistol round. The US uses metric designations for foreign ammo because the case sizing isn't the only difference. Even within "9mm" there are several types which do not function in all 9mm chamber weapons. Metric ammunition is designated in metric to make shooters aware that while the bullet may "fit" it doesn't shoot. 9mm rounds will fit many .40cal mags allowing a 10rd mag to hold more ammo where high cap rounds are illegal for instance. Different dimensions mean different performance and ballistics, with all ammo having some trade in power, range, noise, recoil, weight, etc. Shooting ammo not meant for a gun can be dangerous. Unless you are a well educated shooter stick to the manual.

A 9mm is not equivalent to a .22. When people say ".22" they usually mean a .22lr which was a riffle round and more powerful than a .22 short. The 22 short lacks the power to cycle most semi auto pistols and riffles, and isn't really used outside revolvers, even then its low power makes it unpopular except in vintage use. There's also a .22 Winchester which is larger diameter and won't fit most ".22's." The .22lr is closest to a .223 which is close to a .556