The U.S. Navy Had a Weird, Hand-Cranked Grenade Launcher
But the old-school design set the stage for weapons still in use today
Originally published on May 25, 2015.
Since the very first firearms appeared on the battlefield, weaponeers have looked for ways to send more bullets at the enemy. So when the Pentagon cooked up the 40-millimeter grenade in the late 1950s, American engineers wasted little time in developing plans for even faster firing versions.
In 1962, the U.S. Navy also got into the quick-firing grenade launcher business. The result was an almost anachronistic, hand-cranked grenade launcher that was both easy to operate and relatively light.
The new Mk-18 represented an entirely new class of weapons. While the sailing branch quickly phased out these early grenade-tossing guns, the launcher set the stage for weapons the American military still uses today.
“This weapon, Mk-18 Mod 0, represents the introduction … of a new concept in weaponry,” George Chinn wrote in the fifth and final volume of his definitive work The Machine Gun. “It is the first step past the single-shot M-79 grenade launcher.”
The Army’s M-79 was a common and effective — if crude — grenade launcher. The weapon looked like a sort of oversized sawed-off shotgun. Troops nicknamed the launchers “bloopers” or “thump guns.” A soldier could lob a 40-millimeter high-explosive shell well beyond where they could throw a hand grenade.
But these same rounds in a single-shot launcher made the idea of a small, rapid-firing grenade machine gun possible, according to Chinn. Even better, the Honeywell Corporation had started development of the Mk-18 on its own initiative.
“Honeywell had anticipated the need for a simple, inexpensive and extremely mobile rapid fire weapon … with an effective capability for delivering large quantities of explosive grenades,” Chinn explained.
We don’t know why Honeywell chose a dated design using a manual crank. But weapons like the Gatling and Gardner guns had successfully proven that manually operated mechanisms worked just fine almost a century earlier.
The design did keep the Mk-18 extremely simple. At its heart, the launcher centered around two large gears, one on top of the other. A fiberglass belt held the grenades in place as they moved into position.
The teeth on the sprockets formed a sealed cavity when they lined up. As a sailor cranked the handle, the belted cartridges would feed from one side into the firing chamber and empty shell casings came spitting out the other.
The rotating handle also pulled back and released the firing pin at the right moments. The firing mechanism itself, with a large knob at the end, could be locked in place and doubled as the safety switch.
All of these components sat inside a metal shell. The whole weapon weighed 19 pounds—less than a standard M-60 machine gun. Individual belts generally held 25 grenades. A shooter could fire off the rounds as they could turn the handle.
“[But] single rounds may be fired easily by merely rotating the crank one half revolution,” Chinn noted. “Firing rates as high as 250 rounds per minute may be achieved.”
Two years after starting the project, Honeywell patented the design. A year later, the company began producing the weapons for the Navy. The sailing branch end up buying around 1,200 of the launchers.
With America at war in Southeast Asia, the weapons quickly made their way to Vietnam’s rivers and canals.
Fast-moving patrol boats and heavily armored landing craft sported the guns in addition to a slew of other weapons. In some cases, enterprising crews even strapped the launchers on top of .50-caliber machine guns for more firepower.
The Navy’s elite SEAL teams also bolted Mk-18s onto their specialized support craft. The rapid-firing launchers could help break up ambushes or drive off enemy forces in hot pursuit.
Unfortunately, the guns also had a number of serious flaws.
For one, the belts were specific to the gun. Grenades had to be put through a special machine to link them together. The whole system could jam up if a cartridge ended up misaligned in its loop. Cranking too fast could cause the mechanism to seize.
Far worse, the crude firing pin might fail to strike the primer and set off the propellant. In the field, these issues could be a matter of life and death.
However, Honeywell’s design did show that an automatic grenade launcher could be devastating on the battlefield. Both the Army and the Navy took the basic idea and moved on to true, fully-automatic designs.
While the ground combat branch moved first, developing its own grenade guns at the same time as Honeywell’s team, the Army expected to mount the weapons on helicopters and vehicles. As a result, the launchers generally required an external power source.
In an amusing twist, the Army ended up experimenting with a manually cranked version of one of these designs, dubbed the XM-173. The plan was for ground troops to use the launcher on a tripod.
But as with the Mk-18, the XM-173 quickly became obsolete as a flood of new designs became available. Unlike Honeywell’s design, the Army’s gun never saw combat.
In 1966, the Navy began work on an improved weapon, the Mk-19. Nearly two decades later, the Army decided to pick up the launchers for their own soldiers, too. Today, the guns are used across the services by troops on the ground and on armored vehicles.