because there is a difference between a pistol and a revolver cartridge, even of the same caliber (like say .44 magnum, wits you can both find in the semi-auto DE and a typical magnum revolver).
The .44 magnum ammunition used in the real world in the Desert Eagle is the exact same .44 magnum ammunition used in the real world in my Super Blackhawk. The DE is a semi-automatic, and my Super Blackhawk is a single-action revolver.
There are both semi-automatics and revolvers that fire the .45 ACP (
Automatic Colt Pistol) round, and both semi-autos and revolvers that fire the 9mm round. And http://www.gunsholstersandgear.com/2009/01/03/smith-and-wesson-model-310-night-guard-revolver-in-10mm-and-40-sw/ is a Smith & Wesson revolver that fires 10mm.
There is no difference between a "pistol" cartridge and a "revolver" cartridge unless the cartridges are given diffrent designations. .44 magnum cartridges are .44 magnum cartridges and will fit in any weapon chambered for .44 magnum, 10mm cartridges are 10mm cartridges and will fit any weapon chambered for 10mm.