The Ontos, technically the 106mm Self-propelled Rifle M50, was a light anti-tank vehicle developed in the US in the 1950s. It mounted six 106mm recoilless rifles as its main armament, which could be fired in rapid succession against single targets to guarantee a kill. The US Marines were the only force to use the Ontos operationally, and although their crews consistantly reported excellent results against infantry during the Vietnam War, the Ontos was always considered an "ugly duckling" and was removed from service in 1969.
The Ontos (Greek for "the thing") project was created to be an air transportable tank destroyer capable of being lifted by the cargo aircraft of the 1950s. This limited it to weights between 10 and 20 tonnes, the only other limitation to the design being that it had to use the six-cylinder engine then widely used in the Army's GMC trucks. Allis-Chalmers' won the contract in November 1950, for 1,000 vehicles, while another similar effort led to the M551 Sheridan.
Allis-Chalmers first vehicle, completed in 1952, was based on the running gear of the M56 Scorpion light anti-tank vehicle, mounting a cast steel turret with two arms holding three rifles each. This early model could traverse the turret only about 15 degrees. A second prototype used a new suspension system including new tracks, and a newer turret with about 40 degrees traverse. Only eighteen rounds for the main guns could be carried inside the vehicle due to limited space. Four of the rifles also had 50-caliber spotting rifle attached, firing a round that flew like the 106mm round and gave off a puff of smoke on impact. A single 30-cal M1919A4 machine gun was also carried for anti-infantry use.
The prototype and testing stage was completed by 1953, which is when the Army then cancelled it's order. As an anti-tank vehicle the Ontos had several problems, including a lack of ammunition, rather high profile for such a small vehicle, and the need for the crew to exit the vehicle in order to re-load the guns, making them obvious targets for snipers. The Marines, desperate for any anti-tank support, then ordered 300 for their own use, including three new prototypes with minor changes as the M50A1. Allis-Chalmers then developed fording gear for the Ontos to allow amphibious operations, but this was never accepted for production. The A1 production version first appeared in 1955, and the Marines continued to test the Ontos until it was accepted for service in 1956.
Allis-Chalmers also modified the design as a light APC, but only one prototype was built as the Carrier, Full-Track, Personnel T55. This proved utterly impractical due to the limited room inside, carrying only five infantry and forcing the driver to lie prone. A "stretched" version known as the T56 was also built, and while it held a complete eight-man team, their equipment had to be carried on the outside. Neither was considered very useful.
In 1960 there was a brief study made to replace the Ontos's 106mm rifles with a new 105mm design that included a re-loading system similar to that on a revolver. This project was not accepted. However another upgrade was, replacing the GMC engine with a newer Chrysler V8. Of the 297 vehicles accepted by the Marines, only 176 were converted, the rest being broken up for scrap.
The Ontos was used widely in Viet Nam as a fire support vehicle, and was particularily liked by its crews. They noted that the appearance of an Ontos was usually enough to make the enemy break and run. Nevertheless the Ontos units were deactivated in May 1969, and some the vehicles handed over the an Army Light Infantry Brigade. They used them until they ran out of spare parts, and then removed the turrets and used them as fixed fortifications. Both these and the rest returned from Viet Nam in 1970, and were cut up for scrap, with some of the chassis being sold off as construction vehicles.