The HK P11 is manufactured with five barrels, each loaded with one dart. This design is commonly referred to as a pepper-box design, but the method in which the P11 is loaded remains unknown to most users, requiring the barrel unit to be sent back to Heckler & Koch each time it needs to be reloaded.
The HK P11 is fitted with an electronic firing system, designed to ignite the rocket-fuel-powered dart. This system is powered by a battery pack, located in the grip. This system must remain water tight or the P11 would not fire, which is another clue as to why HK specify that the P11 is sent back to them to be reloaded.
The HK P11 fires a 100mm (3.9in) steel dart. This dart is often referred to as a rocket due to the fact that it is fuelled by rocket fuel with fins reminiscent to those found on a rocket. The use of a dart is designed to be more accurate than a conventional bullet.
The HK P11 is used by several nations for their special forces as well as the Navy forces. Notable users of the P11 include Germany (with the German Commando Frogmen), United Kingdom (Special Boat Service) and the United States (U.S. Special Operations). Previously HK had been reluctant to acknowledge the existence of the P11, having previously designed the Mk I Underwater Defence Gun.
A similar design of Soviet origin, the SPP-1 Underwater Pistol, is a smaller alternative to the P11. The P11, on the otherhand, has five barrels, allowing for more shots to be fired, with the SPP-1 has four barrels.