Heckler & Koch HK4

The HK 4 pocket pistol was first introduced by Heckler & Koch in 1967 making it the first pistol produced by the company. Mass production of this pistol started in 1968 and continued until 1984. There were approximately thirty-eight thousand pistols produced during this time. Serial numbers 10001 to 36550 were for commercial distribution. Twelve thousand pistols were produced for the German police and other government agencies in the .32 ACP (7.65×17mm Browning SR) caliber with the designation P11 and were serial numbered 40001 to 52400.

It is thought that the basic design of the HK 4 is based on that of the Mauser HSc. The HK 4 was designated "4" because the pistol had the ability to be modular in four calibers. This means the frame and slide could accommodate three (3) different centerfire calibers by simply replacing the barrel/return spring assembly and inserting the proper magazine for the caliber of choice. The fourth caliber, .22 Long Rifle (.22 LR), requires the extractor faceplate and firing pin be moved to conform to the proper configuration for rimfire operation in addition to changing the barrel and magazine. The three centerfire calibers that the pistol can use are: .25 ACP (6.35×16mmSR); .32 ACP (7.65×17mm Browning SR); .380 ACP (9×17mm Short). Most standalone pistols were sold in .380 ACP and .32 ACP. All four calibers came as a two-box set with the extra barrels, magazines and a screw driver/cleaning kit in accompanying box.