The HK P11 is a Heckler & Koch pistol designed as an underwater firearm. Since ordinary-shaped rounds are inaccurate and have a very short range when used underwater, this pistol fires steel darts about 10 cm long. It has five barrels, each of which is loaded with a cartridge, giving the gun a pepper-box appearance, and it is electrically ignited from a battery pack in the pistol grip.[1] After firing all five cartridges, the barrel unit must be sent back to its manufacturer for reloading.[2] It is very similar to its predecessor, the Mk 1 Underwater Defense Gun.

This firearm is somewhat bulkier than its Soviet counterpart, the SPP-1 Underwater Pistol, but it has five barrels as opposed to the Soviet firearm that has four.