M50 Ontos – the Vietnam Six-Shooter

Ontos is the Greek word for “Being” or “Thing”, a name that perfectly describes the freaky M50 Ontos, a US support weapon that was bristling with guns.

These guns were 106 mm recoilless rifles, and there were a lot of them – six in fact! Giving the Ontos more firepower than pretty much any other vehicle of its day. At one point, it was even regarded as one of the Marine Corps’ most valuable ground assets.

Initially developed as a utility vehicle, the Ontos would go on to serve in Vietnam, where it was in high demand to clear out enemy positions in cities and jungles.

Origin of the Ontos

The M50 Ontos originated during the Korean War from a project that sought to develop a highly efficient, lightweight, universal chassis.

Experience during that war highlighted the need to give infantry a mobile means of transportation themselves, cargo, and weapons around the battlefield.

The US had produced this quite modern armored personnel carrier/utility vehicle, the M44, by 1945, but it was deemed too large for production.

In 1951, discussions between the Ordnance Department, US Army, and Detroit Arsenal kick started work on a series of small utility vehicles that all shared the same basis. Known as “Infantry Assault Vehicles”, each would be adapted to a specific role, such as a troop transport, cargo transport, and even anti-tank variant.

They were characterised by their small size and emphasis on simplicity and ease of manufacture, as large numbers were expected. Design and manufacture was carried out by Allis-Chalmers, who had previously developed the M4 and M6 high speed tractors.

The armed versions were particularly interesting for their use of recoilless rifles as their main armaments.

Recoilless rifle fired in Korea. These were still relatively new at the time, but were regarded as a game changing weapon.

Recoilless rifles fire projectiles out a barrel like a typical tank gun, but they have significantly reduced recoil. This is achieved by venting the ammunition’s combustion gasses out the rear, rather than imparting all of them on the projectile.

This reduces the recoil, but it also reduces the projectile’s velocity. Therefore, recoilless rifles usually rely on high-explosive anti-tank (HEAT) and high explosive (HE) ammunition as they do not depend on kinetic energy to damage targets.

Recoilless rifles were regarded as game-changing weapons at the time, because thanks to HEAT ammunition, they were capable of dealing with literally any tank then in service.

An M40 recoilless rifle. This type would eventually be used on the M50 Ontos.

Plus, their lower pressures and lack of recoil meant they were very light and could be mounted on vehicles as small as a Jeep. In fact, the French mounted them on Vespa scooters.

With this in mind, it was clear that these weapons were perfect for the Infantry Assault Vehicle. It would give them tank-killing capabilities at a very light weight.

Meanwhile, throughout most of 1951, Caltech conducted a far reaching and highly secretive study that evaluated various components of the US military. Although the study was meant to be a generalised review, particularly of how the US Air Force and Army worked together, it quickly became focused on a war with the Soviets in Europe.

We weren’t joking – France really did mount recoilless rifles on Vespas. This is the Vespa 150 TAP.

One of the many things identified in the study was the potential value in recoilless rifles as a counter to Soviet armor. The study envisioned huge numbers of small, recoilless rifle-armed vehicles known as Ontos, a Greek-originating word with a number of definitions and meanings that mostly relate to the concept of “being” and “existing”.

Coincidentally, what Project Vista was describing with the Ontos was essentially the same as the Infantry Assault Vehicle, despite the two projects being unaware of each other.

These projects eventually met before the end of 1951, giving the Infantry Assault Vehicle’s development and priority a big boost. A family of vehicles were drawn up, all under the Ontos name.

The T55, a six man armored utility vehicle/armored personnel carrier from the Ontos project.

This included two utility vehicles, the T55 and T56, which could carry six and ten men respectively. There was also a number of armed versions based on the same general design: the T164, T165, T166 and T167. Each was armed with varying amounts of recoilless rifles.

All had very simple, wedge-shaped superstructures with thin armor to keep the weight down.

The T55 and T56 were built first, with two pilots of each type being made in total. Testing began in 1952, but both designs were eventually abandoned as they were found to be too small.

The T164 and T167, armed with four and eight recoilless rifles respectively, were never built.

This is the T56. Its hull was extended so it could carry ten men.

The T165 and T166 were built though, with tests also beginning in mid 1952. These vehicles shared essentially the same chassis and superstructure as the T55, with the same engines and running gear too.

Eventually the T166 was knocked out of the running, leaving the T165 as the only surviving vehicle from the Ontos project. It is this vehicle that would become the M50 Ontos.

The T165 was armed with six 105 mm M27 recoilless rifles. These guns were to be mounted on top of the T165 in a semi-traversable mount, and operated from inside by the crew.

T165 pilot model. Note the air-filled rubber tires.

In 1953 the T165’s six M27 rifles were replaced with new and improved M40 106 mm recoilless rifles. These were actually also 105 mm in caliber, but they were designated 106 mm to prevent confusion between it and the older M27.

Work continued over the next year, but the Army would eventually deem the T165 unsuitable due to a number of problems found during its trials. These included poor reliability, and the fact that the rifles had to be reloaded externally by a member of the crew.

Thankfully for the vehicle the US Marine Corps saw interest in it, and placed an order for around 300 in 1955, entering service as the Rifle, Multiple, 106mm, Self-Propelled, M50. Trying saying that ten times!

The M50 Ontos

The M50 Ontos is a rather simple little machine, initially intended to be used as a highly mobile anti-tank platform. It is often said to have been related to the M56 Scorpion, but we have seen no evidence of this.

It has three major parts; the running gear, superstructure, and turret.

Diagram of the M50A1 Ontos.

The superstructure was a wedge shape, with 13 mm (.5 in) of armor all over except the belly, which was 6 mm (.25 in) thick. This was only enough to resist small arms fire and light shrapnel.

Access in and out of the M50 was granted via a large, rubber-sealed double door at the back of the hull.

Inside was a crew of three; a driver at the front left, a gunner in the center and a loader. The loader had no seat to begin with, but eventually they were given one near the rear doors.

At the front of the hull was an Allison XT-90 3-speed transmission, which was connected to the front-mounted final drives. Behind this was the engine. The M50’s engine was a 5 litre (302 cu in) Model 302 petrol straight six that produced 145 hp.

The M50A1, a slightly improved version of the Ontos, was powered by a 5.9 litre (361 cu in) HT-361 petrol V8. This produced 180 hp.

Overall the M50 weighed around (19,000 lbs) fully loaded, and had a top speed of 30 mph.

The running gear consisted of four roadwheels and a drive sprocket. On initial versions the roadwheels had pressurised rubber tires, but in production these were replaced with solid rubber tires.

The M50A1’s HT-361 V8.

Suspension was rubber bushings, controlled via hydraulic shock absorbers and bump stops. For additional structural strength, a channel was bolted to the road wheel swing arms that ran along the outside of the running gear.

The M50 rolled on 20 inch-wide steel-reinforced rubber band tracks. Combined with the M50’s already low weight, these tracks resulted in it having an impressively low ground pressure of just 5 psi.

But of course, the M50 Ontos’ main feature was its wild arrangement of six 106 mm M40 recoilless rifles on top.

The M50’s recoilless rifles could be fire all together, or individually.

The guns were numbered 1 to 6 (starting from the left, viewed from the rear), and could be fired independently, or all together in a devasting barrage.

They were mounted on the Rifle Mount T149E5, with three on each side positioned in triangular clusters. This mount could traverse 40 degrees each side, and elevate +20 degrees and depress -10 degrees, but it was hand operated and did not have any powered controls.

While all of the M50’s recoilless rifles could be removed and fired off the vehicle, guns 2 and 5 were specifically designed for this.

The numbering arrangement of the Ontos’ six guns.

When on the vehicle they were connected to a rather impressive control panel that allowed the gunner to choose which guns were fired, and showed the current status of each gun.

The guns’ status could be ascertained by six numbered lights, each corresponding to the rifles’ particular number mentioned earlier. A bright light indicated that gun was empty, while a dim light indicated that it was loaded.

Below the six lights were another two, marked “E” and “L”. These were “comparison lights”, which served as examples for the gunner to compare the brightness level between empty and loaded.

The Rifle Mount T149E5 that held the 106 mm rifles. They were clamped into the mount, and connected to hydraulic and electrical systems to facilitate remote firing.

When loaded, the guns breeches could be remotely opened and closed by the gunner via a hydraulic system. Similarly, they could be fired remotely with electronic solenoids.

On top of guns 2, 3, 4, and 5 were .50 caliber M8C spotting rifles. These were semi-automatic, and fired tracer rounds to help the gunner establish the target’s range.

However, only the spotting rifles on guns 3 and 4 were used on the vehicle. The rifles on top of guns 2 and 5 were only used if the guns were fired on the ground. In addition, these guns were equipped with telescopic sights for the ground role.

The loader had to load the guns from outside the vehicle. Because of this, M50s would typically identify positions in advance to fall back to for reloads.

The only other armament on the M50 was a .30 caliber M1919 machine gun, mounted above the gunner between the recoilless rifle clusters. This could either be fired remotely from inside, or by the gunner through his hatch.

Reloading the recoilless rifles was quite a hazardous affair, as it required the loader leaving the vehicle to access them and load the rounds. Only 12 rounds were carried by the vehicle, although this number increased to 18 when accounting for the rounds inside the guns.

Four were stored inside, while the remaining eight were stored in a compartment below the hull, accessed externally from the rear.

This rear view of a damaged Ontos shows the external ammunition stowage, below the large doors. The Ontos in this photo has just been hit by a nuclear bomb test.

The M50 in Action

The M50 Ontos’ service life followed a similar path to its stablemate, the M56 Scorpion.

Both vehicles were designed as tank destroyers and deployed to Vietnam, only to find themselves pressed into other roles due to a lack of enemy tanks in that war.

However unlike the 90 mm on the M56, the M50’s six 106 mm recoilless rifles offered tremendous, overwhelming firepower that were actually an asset in the conditions of the Vietnam War.

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These vehicles were deployed to Vietnam with the US Marine Corps in 1965. As outlined in a 1965 Marine Corps document, the Ontos was considered one of the most valuable piece of equipment during a mechanised operation at the time.

USMC Ontos in the Quang Tin Province of Vietnam, 1966. Crews and Marines attached to Ontos units would often sit on top of them in transit, as there was no room anywhere else.

Vietnam was a tough place for any armored vehicle. Thick vegetation, uneven, soft terrain and punishing atmospheric conditions all worked to halt, bog down, and degrade equipment.

Visibility was also very limited, making it easier for the enemy to ambush tanks and other vehicles from concealed positions. With its thin armor the Ontos was particularly vulnerable, but its excellent mobility proved to be very capable in the harsh conditions of the region’s jungles.

Where the M50 Ontos really shined though was with its firepower. Its six recoilless rifles could output a devastating amount of fire down range, and while they weren’t used as anti-tank vehicles, they were great at destroying cover and buildings.

An Ontos at the USMC Combat Base Camp Evans in Vietnam, 1967. Note the protective sleeves over the barrels.

A particularly terrifying round was the M581, which contained 9,600 fléchettes (small dart-like projectiles) that would spread out toward the enemy like a super-sized shotgun. A six-gun barrage of these rounds from a single Ontos launched 58,000 projectiles at the enemy – a truly fearsome weapon.

In general though the M50 Ontos was a vehicle that had to be used and operated very carefully to offset its vulnerabilities and danger to others. It couldn’t directly face off with the enemy due to its thin armor, and was found to work best in an ambush role.

Read More M56 Scorpion – Tiny TD that Could be Dropped out an Airplane

The immense flash produced by its guns was easily spotted by the enemy, so Ontos crews would identify multiple positions they could relocate to during engagements. In addition to this, people nearby had to be careful of the huge backblasts generated by the M50’s recoilless rifles.

This Ontos is firing just one of its recoilless rifles, and is producing quite the cloud.

The three man crew was also quite a disadvantage. The gunner had to command the other two crew members, find targets, fire ranging rounds with the spotting rifles, choose the number of guns needed, track the target and fire.

This was a demanding job that ideally would have been divided up with a dedicated commander.

On the flip side, the enemy quickly learned that the appearance of a lone .50 caliber ranging round nearby meant up to six 106 mm rounds were about to arrive, often resulting in the prompt abandoning of the position.

An Ontos disguised as a bulldozer in 1969. We are not certain why this was done.

The M50’s most important moment came in Hue in early 1968 during the Tet Offensive. In this offensive, NVA and Viet Cong troops assaulted the city of Hue and began intense urban battles against US and South Vietnamese forces.

This old city was very dense and riddled with small streets unsuitable for tanks. While tanks were brought in, the M50 Ontos became a popular sight among friendly forces in the city, where it slipped into the role of a close-range assault gun.

Thanks to its small size, it could easily move through the city’s streets and alleyways and soon became one of the Marines’ most in-demand assets.

M50 Ontos firing on a street in Hue, 1968.

M50s worked closely with Marines to clear streets and buildings, using their guns to punch holes in walls or even bring down entire buildings occupied by enemy forces.

Ultimately the US emerged victorious from the Battle of Hue, and many attribute a significant portion of that success to the M50 Ontos, even over much larger tanks like the M48.

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Toward the end of the 1960s the M50 Ontos’ career wound down as the US Marine Corps left Vietnam. In the 1970s they were removed from service, and most were scrapped. Fortunately a number survive today in museums and private collections.