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Ontos – the Little “Tank” with 6 Cannons

At least in the United States, tanks are almost always named after well-known generals. 

Between World War II and the present, American tanks and armored vehicles got their names from legendary military men like generals William Tecumseh Sherman, John “Blackjack” Pershing, Creighton Abrams and Omar Bradley. 

But during the Vietnam War, a small armored vehicle that wasn’t technically a tank got a moniker fitting its odd and awkward design.

Called the Ontos or “Thing” in Greek, the lethal pint-size contraption featured a small turret that was only partially traversable, on the outside of which were mounted six 105 mm recoilless “rifles” – cannons really – that made it look like something out of a schoolboy’s sketchbook or a low-budget sci-fi movie from the ‘50s. 

Featuring a pick-up truck size chassis and a honking 302 cubic inch (4.9 liter) General Motors engine that thumped out an adrenaline inducing…145 horsepower – about the same as a 1982 Buick Skylark – the Ontos was one of the strangest armored vehicles of all time. 

Sadly, with the exception of military historians and Vietnam vets, these days it’s little more than a historical curiosity. 

However despite being produced in limited numbers, rarely serving in the role for which it was built, being phased out quickly, and having more than its fair share of inherent design flaws, the little cannon platform would go on to make quite a name for itself in Vietnam. 

Design and Development 

With the advent of the Cold War after WWII, tanks became increasingly bigger and heavier. 

Though expensive, complex and difficult to transport, it was thought that future conventional wars would be fought by huge formations of 50, 60 and 70-ton main battle tanks storming toward one another across the plains of Eastern Europe. 

That said, some visionary military leaders recognized the value in supplementing their big tank forces with cheaper, lighter and more mobile vehicles that could be airlifted to out-of-the-way hotspots at the drop of a hat. 

Then once on-site, they could be used for everything from tank busting and reconnaissance to artillery and infantry support. 

Though small and lightly armored, to fulfill these roles the vehicles would need to carry big guns that would at least theoretically make them wily ambush predators capable of knocking out heavier tanks with even bigger guns from long ranges. 

If the doctrine was correct, in some scenarios the combination of speed, agility and firepower would win the day, albeit at the expense of armor protection.  

And as such, Ontos crews were schooled in classic “shoot and scoot” tactics like those used by thin-skinned but heavily gunned German and American tank destroyers from the Second World War like Hetzers and M10s respectively. 

Ontos Tank
Ontos Tank

Technically called the Rifle Multiple 106 mm Self-propelled M50, the Ontos was originally developed as a simple and inexpensive tank killer in the ‘50s. 

But though officially classified as 106 mm, the guns’ actual caliber was 105 mm. 

It’s confusing, but the designation was intended to prevent careless and frugal crews from using leftover M27 recoilless rifle ammo, which the Ontos’ guns replaced.

Unlike traditional tank guns that recoil immediately after firing, the M50’s recoilless rifles, you guessed it, didn’t recoil. 

On standard tank guns the recoil mechanisms are necessary to tame the huge “kicks” experienced after a round is expelled, but they’re complex, expensive and heavy.

To get around this, recoilless rifles vent a portion of the exhaust gases out the breach immediately after firing. 

This results in less pressure inside the tube, which means that barrels can be thinner and lighter. 

On the downside, muzzle velocity and range are decreased, and the exhaust gases can injure or kill anyone standing behind the guns when they’re fired. 

But drawbacks aside, with six guns to regular tanks’ one, each Ontos could lay down fire more quickly, thereby increasing the likelihood of making a kill. 

The original Army specs called for a tracked tank destroyer powered by an existing engine that was light enough to be airlifted by the transport planes of the day.

This last requirement limited vehicle size significantly, and historic photos of Ontos with servicemen beside them show just how tiny they were. 

After a relatively short design phase, American heavy equipment manufacturer Allis-Chalmers was awarded a contract to build 297 vehicles.

The first unit that rolled off the assembly line was based on the running gear of the M56 Scorpion light anti-tank vehicle. 

With a maximum speed of 30 miles per hour (48 km/h) and a range of 110 miles (180 km), the new Ontos was an agile little machine. 

Less than 13 feet (4 m) long, 8 feet 6 inch (2.6 m) wide, about 7 feet (2.1 m) tall at the top of the uppermost gun, each Ontos tipped the scales at 19,000 pounds (9,070 kg), but the interior space was terribly cramped for the crew of three which consisted of a driver, gunner and loader. 

In addition to its six recoilless rifles, each Ontos was equipped with a .30 caliber (7.62 mm) M1919 Browning machine gun for anti-infantry use.  

Though early model’s small cast steel turrets could only traverse about 15 degrees to each side, this was significantly improved on later variants, giving the guns much wider fields of fire without having to move the whole vehicle. 

Due to the cramped cabin, each Ontos could only carry 18 rounds, which meant they were capable of three rounds of firing with two shells left over.

Since Ontos didn’t have traditional scopes, sighting was done with four coaxially-mounted .50 caliber machine guns called spotting rifles. 

Each fired tracer rounds out to 1,500 yards that emitted puffs of white smoke when they hit their targets.

This showed the crew exactly where the big guns were pointing, but it also tended to give away the Ontos’ position. 

Most of the testing and development were carried out at the Aberdeen Proving Ground in Maryland, where the vehicle’s mobility, reliability and effectiveness under tough conditions were assessed.  

The relatively light weight of the M50 made it exceptionally mobile, and pound-for-pound no other vehicle matched its mobility or firepower. 

The test vehicles performed admirably, but by mid-1955 the Army had lost interest and the original order was cancelled. 

The fact was, that in a long-distance slug fest with a behemoth like a Soviet T-55, the pint-size Ontos would stand little chance unless it was able to ambush its prey, get off multiple rounds first, then zoom away out of range. 

Soviet T-55 in Arms museum in Poznan, Poland.
Soviet T-55 in Arms museum in Poznan, Poland. By Radomil, is licensed under CC-BY-SA

In addition, as a dedicated tank killer the Ontos had a host of other problems including a small ammo load and laughably light armor on the front, sides and bottom that made it vulnerable to mines, hand and rocket-propelled grenades and heavy machine guns. 

It was also tall for its size which made it relatively easy to see and hit, but perhaps worst of all, one unlucky crewman actually had to get out of the vehicle to reload the guns after they were fired, which wasn’t exactly a choice job during a firefight.  

Another problem that plagued the Ontos was repeated accidental firings caused by misadjusted firing cables.

This was an easy fix, but with the Army’s canceled order and its inherent flaws laid out for all to see, the Ontos’ future was bleak. 

But as is often the case, what the Army doesn’t want, the Marine Corps will gladly – or grudgingly – take. 

In fact, they just happened to be in the market for a light anti-tank vehicle, and they ordered nearly 300 Ontos that would serve them well in years to come. 

Production ran from 1955 through 1957, and the Marine Corps accepted its first vehicle at the end of October of 1956, nearly a decade before America entered the war in Vietnam. 

Variants 

Even as early as 1960, the drawbacks of the Ontos’ gun system were evident, and designers set about trying to make the loading process quicker and safer. 

One version included an autoloader with a revolver-like drum which made multiple firings between reloads a reality, but the system was deemed too complex and impractical and therefore cancelled. 

After its useful life was over a number gunless variants were also made including a small recovery unit and armored personnel carrier, but Ontos were just too small to do either job well. 

Service

Though they’d go on to cut their teeth in Southeast Asia, Ontoses were first deployed during the Crisis in Lebanon in 1958. 

Since this was a peaceful intervention they didn’t see action, but later during America’s involvement in the Dominican Civil War between 1964 and 65, they were credited with taking out two old Swedish L-60 tanks and a newer French AMX-13 light tank.  

Landsverk L-60
Landsverk L-60. By Wassen, is licensed under CC-BY-SA

Then shortly thereafter with the American buildup in Vietnam, the Marines shipped their Ontos overseas for their trial by fire. 

However, since the terrain didn’t favor heavy armored vehicles and because the NVA and Vietcong specialized in guerilla-style warfare, tanks didn’t play as significant a role in Vietnam as they did in the Second World War. 

Hence, Ontos companies were divided up and attached to other units where they became “Jacks of all trades.”

This presented crews with a number of unique challenges, largely because they’d been trained almost exclusively in anti-tank tactics.

In addition, there was never a designated Marine occupation for Ontos crews.

Instead, teams were cobbled together with volunteers and those looking for a change from the infantry, often with little regard for their skill sets. 

Therefore, there was almost no institutional memory on which Ontos crews could build, which meant that it was largely a learn-on-the-fly gig. 

Now thrust into an environment unconducive to armor, it was up to commanders and crews to perform other roles, namely supporting the infantry with direct fire support.

But thanks to their thin armor, they were often deployed in static positions surrounded by logs and sandbags to protect them from heavy machine guns, artillery and RPGs. 

Ontos often accompanied much heavier M48s, but in many instances they were the only tracked vehicles with big guns capable of crossing pontoon bridges and rice paddies, the latter of which were famous for swallowing heavy American armor. 

Where the Ontos’ versatility and firepower shone the brightest was during the Tet Offensive, in which surprise country-wide attacks were launched between January and September of 1968. 

The offensive was an attempt to foment rebellion among the South Vietnamese population and encourage the United States to scale back or altogether rethink its involvement in the conflict. 

One of the most notable battles during Tet took place in the ancient city of Hue on the Perfume River, when Marines laid siege to the city’s Citadel where NVA troops had wholed up. 

Ontos were brought up to shell the building and to clear enemy snipers secreted away in positions that were impervious to small arms fire. 

At the same time, North Vietnamese forces were battering the Marine outpost at Khe Sanh, and 10 Ontos were transported by MH-53 helicopter to aid the beleaguered defenders. 

Again they performed multiple roles admirably and helped keep the attackers at bay albeit temporarily, in many cases from ranges approaching 1,500 meters.  

In general, Marine grunts loved having Ontos around not only because they packed big punches, but because the sight of them on the battlefield was often enough to make lightly gunned adversaries call off their attack or withdraw altogether.

This definitely wasn’t the case at Hue or Khe Sanh, but stories abounded of enemy soldiers running from abandoned buildings after Ontos spotting rounds flew through windows or doors, because the resulting puff of white smoke meant that a high-explosive 105 mm round was on its way.  

Ontos were capable of firing high-explosive and armor piercing shells, but they were equally effective as anti-personnel weapons when armed with “Beehive” sabot rounds that unleashed thousands of tiny steel flechettes while in flight, which together were capable of hacking down anything in their path and inflicting particularly horrific wounds. 

But despite their success, remaining Ontos units were deactivated in May 1969 largely because spare parts and tracks were becoming scarce, and the inoperable machines were being cannibalized to keep the few working ones up and running. 

All this meant that with the number of usable machines dwindling rapidly, the writing was on the wall – the Ontos’ days were numbered. 

The End of the Ontos

Though Marines typically reported excellent results with their Ontos, by the late ‘60s the stock had dwindled to such lower numbers that the logistical hassle of keeping them maintained and supplied was no longer worth the effort. 

They were officially removed from service in 1969 and quickly forgotten. 

Of the Ontos sent back to the United States, most were sold for scrap, while others were purportedly stripped of their military gear and converted into makeshift construction vehicles like trenchers and bulldozers. 

Thankfully, a few were acquired by military museums and collectors, and even now a few restored examples are on display at places like the National Museum of the Marine Corps in Quantico, Virginia, and the National Museum of Military Vehicles in Dubois, Wyoming. 

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