Ammo goes in, bullets come out. Rifle and handgun ammo may be called cartridges, not shells. Shells an used in shotguns, modern artillery and naval guns. Magnum is a generic gun-word meaning big. It should not be capitalized.
Several firms manufacture magnums.
Semiautomatic, however. doesn't fit too well in 1 col headline. Call it a pistol. Call it a gun if the shot isn't looking. Machine gun is two words. Like deadly mother.