Several folks mentioned the M50 Ontos (Greek for “The Thing”) in comments about the M56 Scorpion. Rightfully so, as the two were somewhat contemporaries and initially conceived to fill the same basic airborne anti-tank requirements. Each represented a different approach, in the days before guided missiles, to providing a heavy anti-tank weapon to the infantry. While the M56 was for all practical purposes a motorized 90mm gun, the M50 used a set of recoilless rifles.
The story of the recoilless rifles themselves deserves detailed treatment in a separate set of posts. Short end of that, in the early 1950s the Army fielded the M40 106mm recoilless rifle (which was really 105mm, but let’s save that for another day shall we?). This large gun fired HEAT projectiles with an armor penetration of 400mm at a range of 3000 yards. But the outfit was far too heavy, at over 450 pounds, for dismounted use. Indeed the outfit pushed the limits of the standard 1/4-ton “jeep.” What the Army wanted was an armored vehicle armed with the M40.
Some sources credit General James Gavin with the idea for the Ontos. However the number of prototype vehicles from the 1950s with recoilless rifles leads me to believe several brains came forward with the idea in parallel. Regardless the basic Ontos chassis started with the T-55 utility vehicle, which was mostly a five-passenger lightly armored vehicle (the T-56 10-passenger “APC” was also offered). Between 1952 and 1955, Allis-Chalmers developed a series of recoilless rifle carriers on the experimental chassis. The T-164 carried four M-40s. The T-165 mounted six. The T-166 featured one rifle in a dis-mountable configuration. And the un-built T-167 was to have eight!
The Army liked the T-165 and proceeded to conduct advanced tests. Dressed out, the T-165 weighed over 8 tons. The Army actually ordered quantity production before canning the project in 1956. Officially the Army cited the vehicle’s high profile, the limited ammunition supply, and external reloading procedures. While not directly competing with the Ontos project, the M56 SPAT then in service weighed less, but left the crew completely exposed. A better way to put it, the Army was already looking at ATGMs to counter the heavy Soviet tanks.
On the other hand, the Marines had a requirement for the old World War II style tank destroyer units. They saw the Ontos as the answer to their needs. Designated M50, the first batches of the Ontos used a six-cylinder truck engine. After a short, two-year, production run, the Marines received just under 300. When the engine proved under-powered, the Marines upgraded about half with an eight-cylinder engine, producing the M50A1. Road speed remained at an impressive 30 mph.
As mentioned, the M50 carried the impressive armament of six M40 Recoilless Rifles, three on each side suspended from a set of arms. A traveling brace on the front hull supported the forward barrel of the lower rifles. The arms connected to a turret, which allowed for an 80 degree traverse, 20 degree elevation, and 10 degree depression. Instead of elaborate sighting arrangements, above the upper four rifles was a .50 caliber spotting machine gun. The .50 caliber rounds followed a similar ballistic path to the big rifles. So the commander simply fired the .50 cals and walked the larger rifles onto the target.
Three men crewed the Ontos – commander, driver, and loader. The rear compartment was so cramped that often the loader sat to the side of the turret. In addition to the six rounds in the rifles, a tray under the crew compartment carried twelve more rounds. A .30 or .50 caliber machine gun on the turret provided close in defense for the vehicle. The armor was enough only for light arms and shrapnel.
The Ontos remained in Marine inventories as the first units deployed to Vietnam in the 1960s. There the “Thing” saw wide service, but met practically no enemy armor. Instead the big 106mm rifles fired high explosive rounds against enemy bunkers or M581 anti-personnel rounds. The later, with a range of around 300 yards, fired 9500 flechette, with legendary effects against troops staging massed assaults. The M50 appeared on news footage during the battle of Hue in 1968:
How’s that for some retro ‘splody?
You might also check out a rather extensive collection of photographs, resources, and links on the Ontos Crewmembers memorial webpage for more on the Ontos in Vietnam.
The Marines began phasing out the Ontos in 1969. Some passed to Army units in Vietnam, who briefly operated them for base defense. But after ten years of service, and no production lines, the Ontos rapidly faded from the picture to be replaced in their original role by TOW carrying jeeps and helicopters.
If I had to rate the Ontos, from the historian’s perspective, I’d call it rather useful but none-the-less an evolutionary dead-end.