When the United States needed an air-transportable tank destroyer in the early 1950s, something that they had been searching for during World War II, a joint effort of the US Army and US Marines brought them to what was called the M50 Ontos. The small, compact, yet powerful enough vehicle was light enough to be airdropped, just as the Marines needed, but why was it still removed from service in 1969?

The Thing

During World War II, when the first attempts to create such a vehicle were made, the limitations in cargo aircraft carrying capacity limited the vehicles’ armor and firepower capabilities, making them no match against the enemy tanks. Come the early 1950s, the US Army and Marines came up with the project known to be the M50 Ontos. In the end, only the Marines would continue using the compact vehicle.

M50’s name “Ontos,” was a Greek word that translates to “thing,” which was the perfect word to describe such a strange vehicle.

105mm Self-propelled Rifle M50A1 Ontos. (US. Army, Public Domain, via Wikimedia Commons)

Producing a vehicle that was compact enough to be airlifted to the battlefield yet powerful enough to match the enemy tanks was indeed a challenge. According to the calculations made by the engineers, the tank destroyer had to be around 10 to 20 tons for it to be air-transportable but still capable enough to fight and survive on a battlefield.

Perfect Match

The M50 was the perfect match with its small, low-profile fully-tracked hull and an enclosed compartment for the crew. To achieve its air-droppability, the engineers had to adjust the armor protection by putting a 13 mm on the hull and just 6.5 mm on the underside to make it light enough.  This would be enough to stop small arms and splinter protection from artillery but not much else.

If this was a bit of a let-down, then you have to check the M50’s main armament: six 105 mm recoilless rifles. To avoid confusion with the M27 recoilless rifle, the M50’s rifles were named 106 mm. A recoilless rifle was installed instead of the conventional anti-tank cannon because the vehicle was so small that recoil from a heavy gun would tear it apart. The Recoilless rifles, on the other hand, balanced out most of the recoil forces by venting propellant gasses out of the weapon’s rear when firing. This clever technique is effective in counteracting the recoil and allows the rifle to fire a larger caliber round compared to conventional weapons of the same size and weight. The barrel can also be thinner to save additional weight.

A marine Ontos crew member relaxes during a break in the fighting at Hue. (National Archives at College Park, Public domain, via Wikimedia Commons)

The rifles could be fired individually or all at the same time. When the six rifles were fired all at once, the back blast of the M50 could blow bricks out of a nearby building and shatter the glass on nearby cars.

The semi-traversable turret where the guns were mounted could rotate up to 40 degrees per side. The guns could be elevated up to 20 degrees and depressed 10 degrees. Ontos used .50 caliber ranging guns on the rifles, with performance similar to the main armament. The ranging round would glow brightly as it approached the target and then emit a white smoke once it hit it. This would be the aiming reference of the gunner.

The M50 Ontos had a top speed of 30 mph and was powered by 5 liters inline 6 GMC engine with 145 HP.

When a Door Closes, a Window Opens Somewhere

After tests were completed and the M50 Ontos was presented to the US Army, they immediately hated what they saw. They thought the vehicle was lightly armored, too small, and the crew would be too cramped up, plus it didn’t carry enough ammunition. However, their main reason was that a crew member had to exit the vehicle every time the rifles would be reloaded. They didn’t want their men exposed to the enemy fire every single time.

As they say, when a door closes, a window opens somewhere. For the Ontos, the window came in the form of the Marines, who were impressed by how the vehicle turned out that they ordered about 300 of them. These vehicles were greatly utilized during the Vietnam War. They were used as a highly mobile fire support platform capable of easily navigating around the jungle battlefield and could provide devastating fire. The CH-53 Stallion helicopter could also lift and drop the M50s in the jungle.

The Viet Cong soon learned that M50s were not to mess with, so they would often retreat at the sight of one.

Ontos leads commandeered vehicles during the Battle for Hue City, 1968. (USMC Archives from Quantico, USACC BY 2.0, via Wikimedia Commons)

Soon enough, the acceptable flaws of the M50 Ontos became too much to bear for the Marines. The large back-blast of the recoilless rifles posed a danger to nearby troops, and the exposed reloading process was just too much of a risk. Its low ammunition storage was unpractical during engagements. Lastly, the M50s thin armor was just no match against larger caliber weapons and even large shrapnel. Anti-tank mines were also much of a threat because the vehicle’s belly armor was thin. They also discovered that the ammunition was prone to explode if the vehicle was damaged.

It was decided to have the M50 Ontos removed from service in 1969, marking the end of its career.