In the early 1950s, the United States military began exploring the idea of a lightweight tank destroyer that could be delivered by air to the battlefield. Similar concepts had been attempted during World War II, but those earlier designs were heavily constrained by the carrying limits of transport aircraft. To reduce weight, armor protection and firepower often had to be sacrificed, leaving many of the vehicles less effective against enemy tanks.
Working jointly, the United States Army and the United States Marine Corps advanced the project that eventually became the M50 Ontos, though the Marine Corps would ultimately be the branch to deploy and operate it. Its name, “Ontos”—the Greek word for “thing”—suited the vehicle’s unusual, compact appearance.
Though relatively small, the M50 was specifically designed to combine mobility with substantial firepower while remaining suitable for air transport into combat areas. Engineers had to work within strict limitations, eventually concluding that the vehicle needed to remain in roughly the 10- to 20-ton range in order to be air-deployable without losing its effectiveness as a battlefield weapon.
The M50 Ontos
The M50 would use a small, low-profile fully-tracked hull and an enclosed compartment for the crew. As it needed to be moved by air, thin armor protection was one of the simplest ways to keep the weight low. Subsequently, the M50 only had 13 mm on the hull (about the same as a WWI Mark 1 tank) and just 6.5 mm on the underside.
But the M50’s showstopper was its main armament of six 106 mm recoilless rifles. Well, actually, they were 105 mm recoilless rifles but were named 106 mm to avoid confusion with the previous M27 recoilless rifle.
The vehicle was so small and light that a conventional anti-tank weapon would recoil too violently, so a recoilless rifle was used instead. Recoilless rifles use a clever technique to balance out most of the recoil forces generated when firing by venting some of the propellant gasses out the rear of the weapon to counteract the recoil. This allows the rifle to fire a much larger caliber round than conventional weapons of a similar size and weight.
Each could be fired individually, or in one single shot. While undergoing trials at the Aberdeen Proving Ground, an M50 fired all six rifles at once, blowing the bricks out of a nearby building while also shattering glass on nearby parked cars.
The guns were mounted to a small semi-traversable turret that could rotate up to 40 degrees on each side. It could elevate the guns up 20 degrees, and depress them 10 degrees. It lacked a complex targeting system, so the M50 used .50 caliber ranging guns on the rifles that had similar ballistic performance to the main armament. The gunner would fire a ranging round, which glowed brightly on its way to the target, emitting a puff of white smoke when it arrived. This would then be used as an aiming reference.
It was powered by a 5 liter inline 6 GMC engine that provided the vehicle with 145 hp. Despite its low power, the M50 was highly mobile and had a top speed of 30 mph, as it weighed just 8,600 kgs.
Tested, then rejected
When the M50 Ontos completed testing in 1955, the US Army rejected it. They considered the vehicle as too lightly armored (only capable of stopping .50 caliber rounds at most), too small, which made it cramped for the crew, and it didn’t carry enough ammunition. But the Army’s main issue with the vehicle was reloading the rifles. To reload, a crew member had to exit the vehicle, exposing themselves to enemy fire. The Army deemed this unacceptable. The US Marines were impressed though and ordered nearly 300.
Impressive service
Although the United States Army chose not to adopt the M50 Ontos, the vehicle found a successful role with the United States Marine Corps, particularly during the Vietnam War. Though originally conceived as a tank destroyer, Marines used it primarily as a highly mobile fire support vehicle. Its six recoilless rifles gave it tremendous striking power, while its speed allowed crews to reposition quickly during combat operations.
The Ontos’s lightweight and compact frame made it especially valuable in environments where heavier armored vehicles struggled to maneuver. It could move through difficult terrain, including jungle areas and narrow routes that were often inaccessible to larger vehicles. Its mobility improved even further with the arrival of the CH-53 Sea Stallion, which gave the Marines the ability to transport the Ontos by air into remote positions and challenging landing zones.
In combat, the vehicle earned a reputation for exceptional firepower. It proved especially effective against bunkers and fortified positions, firing High-Explosive Squash Head rounds capable of breaching walls and defensive structures. It was equally feared when loaded with “beehive” anti-personnel rounds, which released dense volleys of flechettes with devastating effect against infantry. Its battlefield reputation became so strong that enemy forces were said to be reluctant to engage when an Ontos was known to be nearby.
Despite its effectiveness and versatility in combat, the M50 Ontos was retired from service in 1969, bringing its operational career to an end.
End of the road for the “Thing”
The compromises accepted during the Ontos’ development became increasingly obvious in combat. The powerful back-blast from its recoilless rifles created a serious hazard for nearby Marines, while reloading the externally mounted guns exposed the loader to enemy fire. With only a limited supply of ammunition on board, the vehicle was also poorly suited for extended firefights.
Its greatest weakness, however, was protection. The M50’s light armor provided little defense against larger-caliber weapons or even heavy shell fragments, and its thin underside made anti-tank mines especially dangerous. If the vehicle was penetrated, the onboard ammunition could detonate with devastating consequences.
Even with these shortcomings, the Ontos frequently exceeded expectations on the battlefield and earned a fearsome reputation among enemy forces. Col. Stanley S. Hughes, commander of the 1st Marines at Huế, observed, “If any single supporting arm is to be considered more effective than all others, it must be the 106 mm recoilless rifle, especially the M50 Ontos.”