It's not so much the size of the bullet, it's the size of the case. .22 Mag has a bigger case, larger around than the .22LR. Some .22s will shoot .22 Shorts, Longs, and Long Rifle cartridges. Guns like single shots, pumps, and some lever actions. It will be stamped on the barrel, S,L,LR. .22 Mag is designed only to shoot .22 Mag ammo only.