The M50 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 M40 106 mm recoilless rifles as its main armament, which could be fired in rapid succession against single targets to guarantee a kill.
Originally conceived as a fast tank killer, it was employed by US Marines who consistently reported excellent results when used for direct fire support against infantry during the Vietnam War. Its mobility and firepower were proven in numerous battles and operations. Produced in limited numbers and largely expended towards the end of the conflict, the Ontos 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 a weight between 10 and 20 metric tons, 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 was awarded the contract on August 12, 1955, for 297 vehicles.
| Rifle, Multiple 106 mm, Self-propelled, M50 “Ontos” | |
|---|---|
| Type | Tank destroyer |
| Place of origin |  United States |
| Service history | |
| In service | 1956–69 |
| Used by | United States |
| Wars | Vietnam war |
| Production history | |
| Designer | Allis-Chalmers |
| Designed | 1952 |
| Manufacturer | Allis-Chalmers |
| Produced | 1955–57 |
| Number built | 297 |
| Variants | M50A1 |
| Specifications | |
| Caliber | 105 mm |
| Muzzle velocity | 500 m/s |
| Effective range | 2,750 m |
| Maximum range | 7,700 m |
| | |
| Main armament | 6 × M40 recoilless rifles |
| Secondary armament | 1 × .30-caliber machine gun |
| Engine | GM 6-cyl 302 in³ |
Allis-Chalmers' first vehicle, completed in 1952, was based on the running gear of the M56 Scorpion light anti-tank vehicle. The vehicle mounted 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 rifles attached, firing a round that flew like the 106 mm round and gave off a puff of smoke on impact with the target. This meant that the 106 mm recoilless rifles were lined up with the target, and then they would be fired. A single .30 caliber M1919A4 machine gun was also carried for anti-infantry use.
The vehicle was taken to the Aberdeen testing facility where single rifles had been tested. When all six weapons were fired at once, the back blast from the firing knocked bricks out of a nearby building and knocked the rear windows out of several cars. The prototype and testing stage was completed by 1955, at which point the Army canceled its order. As an anti-tank vehicle the Ontos had several problems, including a lack of ammunition, a very 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. Although the Army canceled their order, the Marine Corps were desperate for any anti-tank vehicles they could get, and ordered 297. Production ran from 1955 through 1957. The first vehicle accepted by the Marine Corps was on 31 October 1956.
Several variants were also studied. The Utility Vehicle, Tracked, Infantry, T55 was a light APC, but only two versions of the prototype were built. 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 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. Neither was considered very useful.
In 1960 there was a brief study made to replace the Ontos's 106 mm rifles with a new 105 mm 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 361 in³, V8 engine. Of the 297 vehicles accepted by the Marines, only 176 were converted between 1963 to 1965. This newest version of the Ontos was named Rifle, Multiple 106 mm, Self-propelled, M50A1.
