In 1950, the US Army and US Marine Corps started a joint project to build a light anti-tank vehicle.
It produced the M50 Ontos, a small tracked vehicle with light armor and six recoilless rifles.
The Army rejected the M50, but the Marines embraced it, putting it on the front lines in Vietnam.
They looked nothing like traditional tanks and moved much faster. They had no turret or single large gun, and they were only about 7 feet high and 8 feet wide.
The vehicles were M50 Ontos tank destroyers. Abandoned by the Army during its development, the Ontos was adopted by the Marine Corps in 1956.
Despite its unconventional appearance, it was an important part of the Corps' armored forces during the Vietnam War.
A light, air-transportable tank destroyer
Development of the M50 began in 1950 as a joint Army-Marine Corps project. The idea was to create a light tank destroyer that could be transported by air and quickly deployed to support troops against enemy armor.
To be carried by aircraft, it could weigh only about 10 tons. The result was a model so peculiar that it was given the name "Ontos," which roughly translates to "the thing" in Greek.
It was a small tracked vehicle with no turret and sloped armor. A traditional gun was too much for the vehicle's suspension and hull to handle when fired, so it was replaced with six M40A1C 105 mm recoilless rifles (officially labeled 106 mm to avoid confusion with a different recoilless rifle) that could be fired electronically from inside the vehicle.
The Ontos was capable of firing its rifles one at a time, in pairs, or all at once, and its main guns could be lowered 10 degrees, raised 20 degrees, or moved 40 degrees to the left or right. Four of the recoilless rifles were mounted with .50-caliber M8C spotting rifles that fired bright tracer rounds at targets to ensure the gunner had the correct range.
After seeing it tested, the Army rejected it outright.
The Army thought it was too small for its three-man crew, that its armor was too light (it was mostly unable to stop anything larger than .50-caliber rounds), and that forcing the loader to exit the vehicle to reload the rifles defeated the purpose of an armored vehicle.
The Army canceled its orders and instead pursued the M56 Scorpion. But the Marines pressed on, ordering 297 of them.
Excellent at fire support
The M50's first deployment was to Lebanon during the 1958 crisis.
Its only kills against enemy armor came during the US intervention in the Dominican Civil War in 1965, when it destroyed a French-built AMX-13 light tank and a Swedish-built L-60 light tank.
The M50 saw real combat in Vietnam. Since it was lighter than a tank, it could traverse the jungle relatively easily and could be carried by CH-53 Sea Stallion helicopters.
The North Vietnamese Army and Viet Cong had almost no armor of their own, but the Ontos was an excellent fire-support vehicle. It fired high-explosive shaped charges into fixed positions and "beehive" anti-personnel rounds - which were packed with thousands of sharp steel projectiles - at charging enemy troops.
The M50s quickly gained a reputation as good for static defense. But in 1968, they were sent on offense.
North Vietnamese Army and Viet Cong forces had dug into South Vietnamese cities during the Tet Offensive and were difficult to dislodge. The Ontos was brought in to root them out.
It proved a superb urban-combat vehicle. At ranges of 300 to 500 yards, the recoilless rifles could knock holes in buildings for friendly infantry to enter or destroy a fixed enemy position in seconds.
Its speed and maneuverability meant it could conduct "shoot and scoot" attacks, with it arriving quickly, firing, and then retreating to safety to reload.
"If any single supporting arm is to be considered more effective than all others, it must be the 106 mm recoilless rifle, especially the M50 Ontos," Col. Stanley S. Hughes, the commander of the 1st Marines at Hue, said after the battle.
The M50s also proved their worth at Khe Sanh, repelling numerous human-wave attacks.
But the M50s did suffer from undeniable limitations.
It could carry only 24 rounds for its rifles, far fewer than most other tanks. Standing behind an M50 as it fired its recoilless rifles was also very hazardous because of their backblast.
Its biggest problem was its thin armor, especially on the bottom. Anti-tank mines, used to great effect by the Viet Cong, blasted through the M50s easily.
The ammo-storage setup, which put spare shells at the bottom of the vehicle beneath the rear door, didn't help either. There was a high risk of secondary explosions after running over a mine or getting hit by rocket-propelled grenades.
The cramped interior, coupled with constant enemy fire in Hue, also took a toll on the crews, who still had to exit to reload. Then-Lt. Col. Ernest C. Cheatham said during the battle that when crews "come out of those tanks ... they looked like they were punch drunk."
The Corps' limited order of M50s also meant that as spare parts dwindled, Marines had to cannibalize some to keep others going.
By 1970, the M50s had been taken out of service. The ones that remained were sent home, where most were scrapped.
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