The Mk 18 Mod 0 was a 40x46mm grenade launcher used by the United States Navy during the Vietnam War and also the last known hand crank operated firearm since the Gatling gun.

The Mk 18 Mod 0 featured a pistol grip, iron sights, and a control knob with three different settings: Safe, Load, and Fire.

The only hand-cranked weapon since the Gatling Gun, two rounds were loaded and fired for every complete rotation, while spent cases were reinserted into the belt.

The US Army mounted some on M151A1 Jeeps for patrols; their primary users were the so-called "river rats" and the SEALs.

[citation needed] Armored Troop Carrier (LCM)s of the Mobile Riverine Force usually mounted two Mk 18s.