Heckler & Koch HK4
The HK4 is a blow-back operated unlocked-breech semiautomatic pistol with a double-action trigger and an external hammer. Its name is derived from the tool-free interchangeability of the four barrels in calibers 9mm Browning Short, 7.65mm Browning, 6.35mm Browning and .22 Long Rifle. The gun cons…