(Only available in .25 and older, with the exception of Sub Rosa Underground.) Edit
The Magnum is a deadly, yet costly, weapon available from the in-game gun stores. Known for having an insane amount of recoil, most players steer clear of it, going for more stable weapons.
Although the game is in Alpha, the model for the Magnum is highly inaccurate, looking more like a Desert Eagle. If this was the case, the ammunition would also need to be changed, as the Desert Eagle is a .50 caliber handgun (Also comes in .44 and .357 Magnum).
The Magnum is mentioned throughout the MAGNUM Owner's Manual.