Mk 1 Underwater Defense Gun

The Mk 1 Underwater Defense Gun, also called the M1 Underwater Defense Gun or Underwater Defense Gun Mark 1 Mod 0, is an underwater firearm developed by the United States in the Cold War. It fires a special 4.25 inch metal dart instead of a conventional bullet. Work began in the 1960s under project TDP 3801, much of it at the Naval Surface Warfare Center White Oak Laboratory in Maryland, after rocket-powered designs proved expensive and inaccurate. The gun is a double-action-only pepperbox with a six-round removable cylinder magazine and an aluminum frame. Its Mark 59 cartridge holds a tungsten dart and keeps propellant gases inside the case, which removes muzzle blast and flash. Users found it bulky and heavy, and the Heckler and Koch P11 replaced it from 1976.