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 U.S. Army ultimately decided against adopting the M50 Ontos, the vehicle went on to serve effectively with the United States Marine Corps, most notably during the Vietnam War. While it had been designed as a tank destroyer, the Marines employed it chiefly as a fast-moving fire support platform. Armed with six recoilless rifles, it delivered immense firepower, and its mobility enabled crews to shift positions rapidly during combat.
Thanks to its small size and light weight, the Ontos performed well in places where larger armored vehicles had difficulty operating. It was capable of traveling through dense jungle and along narrow routes that were often impassable for heavier equipment. Its operational flexibility increased even more after the introduction of the CH-53 Sea Stallion, which allowed Marines to airlift the Ontos into remote locations and difficult landing zones.
On the battlefield, the Ontos became known for its formidable offensive capability. It was particularly effective against bunkers and other fortified defenses, using High-Explosive Squash Head (HESH) rounds to break through walls and protective structures. When equipped with “beehive” anti-personnel ammunition, it could unleash dense clouds of flechettes that were highly destructive against enemy infantry. The vehicle gained such a fearsome reputation that opposing forces were reportedly hesitant to attack when an Ontos was believed to be operating nearby.
Even with its proven combat performance and adaptability, the M50 Ontos was withdrawn from service in 1969, marking the conclusion of its operational career.
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.”