The Marine Corps’ Six-Barreled Monster

It looked like someone had taken a tiny armored vehicle and strapped six enormous rifles to the outside.

That description isn't far from the truth.

The M50 Ontos was one of the strangest armored fighting vehicles ever operated by the United States Marine Corps. Small, lightly armored and carrying six 106mm recoilless rifles, the Ontos was originally designed as a highly mobile tank killer.

Instead, Marines discovered that it excelled at something entirely different.

In Vietnam, the Ontos became a devastating weapon against bunkers, buildings and enemy troops—and earned a reputation far greater than its diminutive size suggested.

Its name was appropriate.

Ontos is Greek for “the thing.”

And there really wasn't anything else quite like it.

Built to Kill Tanks

The Ontos was conceived during the early years of the Cold War.

Military planners worried that American forces could someday face enormous formations of Soviet tanks. They wanted a lightweight vehicle capable of destroying armor while remaining small enough to move quickly and, ideally, be transported by aircraft.

The result was an unusual tracked vehicle built around firepower rather than armor.

The Ontos weighed roughly nine tons and carried a crew of three. Its armor offered protection primarily against small-arms fire and fragments.

But its defining feature was impossible to miss.

Mounted outside its small turret were six M40 106mm recoilless rifles—three on each side.

Despite the 106mm designation, the weapons were actually 105mm. The designation helped prevent ammunition from being confused with ammunition intended for an earlier 105mm recoilless rifle.

The Ontos could fire its weapons individually or in rapid succession.

For a vehicle so small, the amount of firepower was extraordinary.

Six Rifles and a Very Small Vehicle

The M40 recoilless rifle used ammunition designed to defeat armored vehicles and fortified positions.

Each rifle also had a .50-caliber spotting rifle mounted with it. The spotting weapon fired a round whose trajectory closely matched that of the main weapon.

The gunner could fire the spotting rifle first.

If the spotting round struck the target, the 106mm weapon was likely to hit as well.

Then came the main round.

The recoilless rifle worked by venting propellant gases backward, dramatically reducing recoil. That allowed a relatively small vehicle like the Ontos to carry weapons that otherwise would have required a much heavier mounting.

But there was a consequence.

The backblast was enormous.

Anyone standing behind an Ontos when it fired could be seriously injured or killed, and firing could throw up a huge cloud of dust and debris that immediately revealed the vehicle's position.

The Ontos could hit hard.

Subtlety was not one of its strengths.

The Army Didn't Want It

Ironically, the service originally interested in the concept ultimately decided against it.

The U.S. Army declined to adopt the Ontos.

The Marine Corps saw things differently.

For Marines, weight and mobility mattered enormously. Equipment had to be transported aboard ships, moved ashore during amphibious operations and operate with expeditionary forces.

A compact tracked vehicle carrying enormous firepower had obvious possibilities.

The Marine Corps adopted the M50 during the 1950s, and production vehicles began entering service later in the decade.

Eventually, nearly 300 Ontos vehicles were produced.

Their defining combat experience would come in Southeast Asia.

The Ontos Goes to Vietnam

By the time the United States became deeply involved in Vietnam, the massive Soviet tank formations the Ontos had been designed to fight were nowhere to be found.

There were relatively few enemy tanks for it to destroy.

On paper, that might have made the Ontos obsolete.

Marines found another use for it.

Vietnam was filled with fortified positions, bunkers, tree lines and buildings occupied by enemy troops.

Against those targets, six recoilless rifles were devastating.

High-explosive and anti-personnel ammunition transformed the Ontos from a tank destroyer into something closer to a mobile assault gun.

The vehicle could move with Marine infantry and deliver enormous direct fire against positions that were difficult to eliminate with rifles and machine guns.

One particular type of ammunition made the Ontos especially feared.

A Giant Shotgun

The M40 recoilless rifle could fire an anti-personnel round packed with thousands of small projectiles.

When fired, it essentially transformed the 106mm rifle into an enormous shotgun.

At close range, the effect against exposed infantry could be devastating.

Marine Ontos crews learned to use the vehicle aggressively.

Enemy troops reportedly learned to recognize the distinctive appearance of the Ontos and the destructive power it could deliver.

But the vehicle had serious weaknesses.

Its armor was thin.

Its ammunition capacity was limited.

And perhaps its greatest problem appeared whenever the six rifles needed to be reloaded.

The Crew Had to Get Out

The Ontos carried only a limited supply of 106mm ammunition.

Worse, most of the rifles could not simply be reloaded safely from inside the vehicle.

A crew member had to leave the protection of the hull and reload the externally mounted weapons.

In combat, that could be extremely dangerous.

An Ontos crew might unleash several rounds against an enemy position, only to find itself exposed while preparing to fire again.

The vehicle therefore depended heavily on movement, positioning and cooperation with infantry.

It was not a tank.

Trying to use it like one could be fatal.

But when Marines used the Ontos for what it did best—providing sudden, overwhelming direct fire—it could be remarkably effective.

The Battle of Hue

Perhaps the Ontos' most famous moment came during the Battle of Hue in 1968.

During the Tet Offensive, North Vietnamese and Viet Cong forces seized much of the ancient city of Hue.

Marines found themselves engaged in brutal urban combat.

Buildings became fortresses.

Enemy troops fired from windows, walls and prepared positions. Marines sometimes had to clear the city room by room and b