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By Joseph Trevithick,War Is Boring

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.

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Aerojet’s original belt-fed design. U.S. Army photo. At top—U.S. Marine Corps officers listen to a presentation about the XM-174. Marine Corps photo

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.”

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“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.”

With this data in hand, the Army sent 10 guns and a supply of magazines to units in Vietnam. The ground combat branch encouraged soldiers to take the weapons out on foot patrols.

But successes in the laboratory didn’t translate to success in combat.

Troops quickly reported major problems with the XM-174. For one, the magazine turned out to be extremely fragile. Pieces often broke, and small dents could jam the whole system.

To fix the drum, soldiers had to completely take it apart. Dissasemble one in combat? No thanks. During three and a half months of field tests, troops threw out 75 busted magazines.

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The version of the XM-174 with a buttstock

While the weapon wasn’t prohibitively heavy, it didn’t have a shoulder sling and was onerous to lug around. Soldiers could rig an M-16 sling to the XM-174, but it was still unwieldy.

The 1st Battalion, 27th Infantry Division was the only unit that participated in the Vietnam evaluation. Within two months, soldiers took the XM-174 along on fewer than one third of their missions. The 1-27 was a “leg” infantry battalion, meaning it didn’t have vehicles to help carry the load.

Far more damning, the experimental grenade launcher had a serious safety problem. Troops discovered that the safety switch could easily move into the “fire” position … on its own.

“In one instance, while a gunner was sitting on top of a [sic] personnel carrier with the weapon on his lap, the selector was brushed from SAFE to FIRE,” the evaluators in Vietnam noted. “In this instance, two rounds discharged into the road embankment.”

Thankfully, the 40-millimeter grenades didn’t explode, because they hadn’t traveled far enough—a safety feature built into the ammunition itself. But in an another freak accident, a soldier lost an eye after a shell casing hit him in the face.

While the Army was still relatively pleased with the results coming out of Vietnam, the XM-174 was clearly not suitable for individual troops. Then other problems started cropping up.

The U.S. Marine Corps and Air Force tested the launcher on armored vehicles and helicopters. Those services had similar complaints about the system. Most notably, the gun could only toss grenades out to around 400 meters. While the XM-174’s explosive rounds were deadly, traditional bullet-firing machine guns had much greater range.

Aerojet never brought back the original design’s belt-fed system to help solve the magazine problem. But to help boost range, the company developed a new variant—dubbed the XM-174E3—that shot rocket-propelled grenades.

Yet, this complicated solution was too little, too late. Another grenade launcher was on its way.

While the Army fiddled with the XM-174, the U.S. Navy developed the vehicle- and tripod-mounted Mk 19. It chucked the faster-flying 40-millimeter grenades, and was already proving its worth in Southeast Asia.

By the early 1980s, the crew-served Mk 19 became the standard fully-automatic grenade launcher across the services. In addition, the Marines today use the MGL semi-automatic grenade launcher?—?first developed for the South African military in the 1980s.

As late as 1976, Aerojet was still trying to find a buyer for the XM-174. The company’s sales literature pitched the design as a nimble weapon for the individual soldier. The firm even sent out a promotional flier depicting the gun with a shoulder sling.

But despite what you might see in action movies, most armies aren’t interested in soldiers toting their own automatic grenade launchers.

This piece was originally published by War Is Boring