 The submachine gun used by Gordon Freeman, the Combine, and  Resistance Fighters in the game appears to be based on the Heckler &  Koch MP7 Personal Defense Weapon. The real HK MP7 is small enough  that it can be easily concealed, carried in a holster, and fired as  a pistol due to its low recoil. The butt can also be extended in  order to allow the weapon to be fired as a rifle. In fully automatic  mode, the weapon can be used as a submachine gun, as is the case in  the game.