Thanks for all the great information, but I'm still a bit confused. You said the .22 WRF may be fired in a .22 WMR but not vice versa. Then the last comment was .22 Magnum Rim Fire and .22 Winchester Magnum Rimfire are two names for the same cartridge. This leads me to believe that I can use WMR ammo in my MRF pistol (my pistol is stamped 22 M.R.F. CTG). What am I missing here. If I can't fire the WMR ammo in this pistol where can I find this older type of MRF ordnance. Once again thanks for taking the time to explain this.