Ontos, officially the Rifle, Multiple 106 mm, Self-propelled, M50, was an American light armored tracked anti-tank vehicle developed in the 1950s.

It mounted six 106 mm manually loaded M40 recoilless rifles as its main armament, which could be fired in rapid succession against single targets to increase the probability of a kill.

The Marines consistently reported excellent results when they used the Ontos for direct fire support against infantry in numerous battles and operations during the Vietnam War.

The Ontos (Greek for "thing"[1]) project was created to be an air transportable tank destroyer capable of being lifted by the cargo aircraft of the 1950s.

After a number of design and engineering meetings from March through October 1951, manufacturer Allis-Chalmers was awarded an initial contract to produce 14 pilot vehicles, encompassing 6 variants.

The Utility Vehicle, Tracked, Infantry, T55 was a light Armored personnel carrier (APC), but only two versions of the prototype were built.

It proved impractical due to the limited room inside, carrying only five infantry and forcing the driver to lie prone.

A "stretched" version known as the Utility Vehicle, Tracked, Infantry, T56 was also built, and while it held a complete eight-man team, their equipment had to be carried on the outside.

The Ontos was therefore more widely used by the US Marines for direct fire support for the infantry in combat, a role that was never emphasized in training or doctrine.

In the Battle of Huế, Colonel Stanley S. Hughes felt the Ontos was the most effective of all Marine supporting arms.

In Operation Desoto, the introduction of the large CH-53 Sea Stallion helicopter made possible moving a platoon 25 miles (40 km) south of Quảng Ngãi City carrying Ontos in slings underneath the aircraft.