WMR and MRF are the same thing. The maker of your gun used MRF to designate "Magnum Rim Fire", same shell as the WMR "Winchester Magnum Rimfire". The .22 WRF is an older, pretty much obsolete round that operates at lower pressures then the magnum shell, and can be used in a gun chambered for the magnum, just not the other way around. Sort of like shooting 38spl's in a 357.. Hope that helps.

Larry