Since ordinary-shaped rounds are inaccurate and have a very short range when used underwater, this pistol fires steel darts about 10 centimetres (3.9 in) 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. After firing all five cartridges, the barrel unit must be sent back to its manufacturer for reloading. It is very similar to its predecessor, the Mk 1 Underwater Defense Gun. In the past, Heckler & Koch has denied knowledge of its existence.
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 which has four. However, the SPP-1 does not need to be sent back to the manufacturer to be reloaded.