Running around the battlefield with a rapid-firing, grenade-lobbing machine gun sounds like something Arnold Schwarzenegger would do in a movie. But in the 1960s, the U.S. Army experimented with one such weapon.
After introducing the crude but functional M-79 in the late 1950s, the ground combat branch saw fully automatic, 40-millimeter grenade launchers as the next logical step.
So the Army began testing the XM-174, which soldiers could schlep around and fire like a handheld machine gun.
Today, the U.S. military uses a different automatic grenade launcher — the Mk 19. But this weapon uses a tripod or a vehicle mount … and for good reason. The XM-174 was awkward, clumsy and dangerous to friendly troops.
In 1964, American advisers in Vietnam asked the Army for a rapid-firing grenade launcher to help beat back communist insurgents. Two years later, Army evaluators settled on the XM-174 — one of four different designs — as a possible basic infantry weapon.
The “requirement was … based on upon requests from the field … for a weapon which could be vehicle mounted and provide a high volume of area fire,” a contemporary Army report explained. “User tests eliminated all but the XM-174 automatic, low-velocity launcher.”
The Aerojet Ordnance Manufacturing Company built the weapon, which looked like a combination of the World War II-era .30-caliber Browning machine gun and the standard M-79 “bloop gun.”
This new “low-velocity” launcher fired the same 40-millimeter ammunition as the “blooper,” and did so at the relatively slow speed of 250 feet per second.
At the time, the U.S. military’s sole fully-auto grenade launcher fired “high-velocity” rounds — which could travel much faster. But the launcher needed to be stronger and heavier to handle the added recoil. Due to its bulk, the Army only fitted this weapon onto early gunship helicopters.
But if the Army used low-velocity rounds, the XM-174 could be significantly lighter than previous designs. Aerojet’s initial cannon weighed only 13 pounds. A typical M-60 machine gun was 10 pounds heavier.
At first, the Army expected to put the XM-174 on vehicles. If soldiers were to use it when dismounted, they’d need a tripod. The Army even rejected a launcher that was more akin to an automatic rifle, complete with a butt stock, bipod and five-round magazine.
But during demonstrations, the lightweight designs did eventually change the evaluators’ minds. With the low-recoil ammo and a slow rate of fire, soldiers could shoot the XM-174 on the move and “from the hip.”
In response, Aerojet modified the design to make it easier for soldiers to carry the ammunition. Instead of a loose belt, the company cooked up a self-contained 12-round drum.
But the weapon was still pretty heavy. For starters, when the gun’s ovular drum magazine was fully loaded, it weighed almost as much as the gun itself. With all that added weight concentrated in a single spot and hanging off to one side, Aerojet’s launcher became frustrating to handle.
“The weight of a loaded magazine … influenced the use of the XM-174 in foot-mobile operations,” an official review stated. “The weapon became especially unmanageable when the gunner was required to travel through deep mud and water and through thick jungle undergrowth.”
“Persons experience[d] carrying a loaded XM-174 … as heavy and awkward,” an Army report stated after tests at the Aberdeen Proving Ground in Maryland. “But an individual’s loss of speed and maneuverability are no greater than if he were carrying a loaded M-60 machine gun.”