Greg Nutt posted a photo:

Ontos, officially the Rifle, Multiple 106 mm, Self-propelled, M50, was a U.S. light armored tracked anti-tank vehicle developed in the 1950s, a fast tank killer for airborne forces.
It mounted six 105 mm M40 recoilless rifles as its main armament, which could be fired in rapid succession against single targets to guarantee a kill. It was produced in limited numbers for the U.S. Marines after the U.S. Army lost interest in the project. 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 American stock of Ontos was largely expended towards the end of the conflict and the Ontos was removed from service in 1969.

The Ontos (Greek for "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 Ontos also had to use the six-cylinder engine then widely used in the Army's GMC trucks. Allis-Chalmers was awarded a contract on August 12, 1955, for 297 vehicles.

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, new tracks, and a newer turret with about 40 degrees traverse. The vehicle could carry only eighteen rounds for the main guns inside the vehicle due to limited space. Four of the rifles also had .50 caliber coaxial spotting rifles attached, each of which fired a round with the same trajectory as the 106 mm round, and that gave off a puff of smoke on impact. The spotting rifles were used to line up the 106 mm recoilless rifles with the target. The Ontos also carried a single .30 caliber M1919A4 machine gun for anti-infantry use.

The vehicle was t