[Lenkers]

Look what I just found:

Handfeuerwaffe und Munition hierfür

Flashless electric Firearm and Ammunition therefore

There are a few interesting things:
The german patent was filed in 1969 and issued (visible to the public) in 2008. Secrecy was obviously very important.
Furthermor the (german) patent doesn't even mention underwater applications. The only reference: "For special operations e.g. frogmen".
Low muzzle flash and low noise are descriped as the outstanding features.
The barrels are probably made of glass reinforced polymer.

 

[G3Kurz]

Look what I just found:

Handfeuerwaffe und Munition hierfür

Flashless electric Firearm and Ammunition therefore

There are a few interesting things:
The german patent was filed in 1969 and issued (visible to the public) in 2008. Secrecy was obviously very important.
Furthermor the (german) patent doesn't even mention underwater applications. The only reference: "For special operations e.g. frogmen".
Low muzzle flash and low noise are descriped as the outstanding features.
The barrels are probably made of glass reinforced polymer.

Interesting find Lenkers.
The P11 had above water ammo (.30 cal bullets - AP, frangible ["sparkless"] and lead core) as well as the underwater ammo ("darts") loaded "bundles". Each 5-round bundle quickly attached to the firing grip for use. It was more than just a weapon for combat divers.

The names on the patent are some of the early designers at HK GmbH in the later 60-80's - Moeller, Brandl. Moeller later became the R&D Director and was famous for opposing things that made sales sense (building the Glock 17 for example) over items that looked innovative (more roller-locked guns, a GMG firing from a drum). Siedel was one of the original founders of H&K along with Koch and Heckler.

The P11 bundle still carries a NATO confidential classification which is why you don't see them in museums or collections. The 50 that were in the US were turned in and destroyed.

G3Kurz