Gun Wiki

The MK 18 Mod 0 was a 40mm grenade launcher used by the United States Navy during the Vietnam War. It was the last known hand-cranked firearm since the Gatling gun.

Design Details[]

The Mk 18 Mod 0 is a belt-fed, manually-operated grenade launcher designed for use on a low tripod, to attack targets with high-explosive fragmenting grenades. It fires with a variable cyclic rate of up to 250 rounds per minute at ranges of 328 to 2,200 yards. Ammunition belts are fed into the weapon from 24- or 48-round ammunition boxes. The belts themselves were hand-loaded into the weapon and could be re-used up to five times.

It uses a rotary split breech, which allows for straight through feeding of ammunition, eliminating the need for a reciprocating bolt, extraction of ammunition from a belt, extraction from a chamber, and ejection for the weapon. While the split breech concept was nothing new, the Mk 18 Mod 0 has some unique characteristics based on a mechanism that allows the top rotor to move vertically and synchronize the bottom rotor. The split breech mechanism allowed the weapon to be light and simple, especially with the 40mm grenade's high-low system in mind.

A three-position knob is located at the rear of the weapon for "safe", "fire", and "load" controls. Two rounds are fired during each full crank revolution with loading occurring at the 3 and 9 o'clock positions of the crank, and firing at 6 and 12.

The Mk 18 Mod 0 could be mounted on M2HB, M60, and M1919 Browning tripods or a pintle mount. However, the weapon cannot fire without such a mount.

Test results had shown that the Mk 18 Mod 0 would fire 5,000 rounds without failure from mechanical wear. But during the evaluation, the weapon's performance during the first 3,000 rounds proved unsatisfactory from high stoppage. This stoppage issue is due to faulty ammunition belting, inadequate firing pin positioning, and the cycling action of the weapon being rough at higher rates of fire. It was also recommended that the firing mechanism be modified to prevent unintentional firings during movement of the firing pin from the safe to the firing position.


Initial development on the Mk 18 Mod 0 began in late 1962, and Honeywell filed the first patent application for it in 1964. Production began in 1965, and ended in the early 1970s. During that time period, approximately 1,200 units were manufactured. Despite having been declared obsolete with the introduction of the Mk 19 grenade launcher, the Navy ordered more than 500 Mk 18 Mod 0s, developed by Honeywell under contracts of more than $750,000.

The Mk 18 Mod 0 was primarily used on small boats and in fixed positions such as bunkers, used by the so-called "river rats" and the U.S. Navy SEALs.