My understanding when I see the term .350 Rigby Magnun is as Huvius stated, that means for the .350 Rigby Magnum Rimless cartridge with the 220 grn bullet loaded to 2600fps. They also made the rimmed Rigby .350 #2 which was the .400/.350 case loaded with the 220 grn bullets to the same 2600 fps instead of using the standard .310 grn bullet for the .400/.350. The rimless cartridge has more taper in the case as well. My Heinrich Scherping Double rifle..circa 1910 is marked .350 Rigby Magnum # 2 as original Rigby ammo boxes for it are marked "Rigby's .350 bore #2 High velocity Nitro Cartridges".

m4220