One of the most unique of the fighting vehicles to emerge from the Vietnam War-era (1955-1975) was the M50 "Ontos". Its name translated in the Greek to mean "thing" which was an appropriate title for the diminutive machine which was unlike any combat vehicle seen to that point. The vehicle utilized a three-man crew in a compact, angled hull superstructure which sported 6 x 106mm recoilless rifles overhead. The type was devised as an air-transportable tank destroyer but found better use in the anti-infantry role during the Vietnam conflict. The United States Marine Corps became its primary user and Allis Chalmers manufactured it from 1955 to 1957 to the tune of 297 units.
The M50 was initially born through five T156 pilot vehicles that each fitted recoilless rifles of different calibers for testing purposes. Twenty-four more vehicles followed for additional trials and these took on the standardized 106mm rifles. The program then graduated to produce the T156E2 pilot model which became the production-quality M50. The USMC initially fielded their Ontos vehicles in October of 1956.
Engineers attempted to create the most compact form possible for the combat vehicle while also preserving crew survivability. This included use of heavily-sloped sides where armor could be thinner but still provide the necessary ballistics protection. This did, however, reduce the internal cabin space which made conditions for the three crewmembers rather tight. The armor was only ever suitable against small arms fire and perhaps artillery spray and little else. The driver sat front-left with the engine to his right.
The weapon of choice for the M50 became the M40 Recoilless Rifle. This weapon appeared during the mid-1950s and saw extensive service in the Vietnam War and beyond. It fired several types of 106x607mmR projectiles that included HEAT and HEAP ("High-Explosive, Anti-Tank" and "High-Explosive, Armor-Piercing" respectively). Its design was such that recoil could be reduced by skillful expelling of propellant gasses during the moment of ignition. This produced a forward motion to counteract the rearward force of the gun - thusly reducing recoil. No complex recoil mechanism was therefore needed and the weapon could remain light in its construction. The M40 was specially designed for anti-tank warfare and initially utilized as a ground-based, crew-served system with a maximum range of 6,850 meters. HE and other anti-personnel rounds broadened its tactical capabilities beyond just tank warfare.
For the M50, six 106mm rifles were installed in groups of three onto a shallow-profile turret to either upper hull side. The two top rifle installations were fitted side-by-side with the third in a lower setting. Coupled to some of the rifles were Remington M8C 0.50 caliber spotting rifles used for training the recoilless weapons. The machine guns were fitted to the two outermost rifles (numbered "2" and "5" if counting the collection from left-to-right from the rear of the vehicle) and the two innermost rifles. These guns were afforded 80 rounds of 0.50 caliber ammunition. The turret held limited side-to-side traversal and elevation value. The spotting rifles were used after optical targeting was accomplished by the gunner. Coupled together, this provided some degree of accuracy for the rifles when fired. Self-defense was through a single 0.30 caliber Browning M1919A4 air-cooled machine gun.
The high placement of the weapons allowed for the M50 crew to press up behind a fortified or earthen wall for protection and still fire its weapons overhead. Additionally, the recoilless rifles retained their crew-serve capabilities and could be quickly detached from the vehicle and operated as normal - perfect for arranging an ambush or supporting infantry in other ways. 18 x 106mm projectiles were carried aboard for reload fire.
The little vehicle was initially given a General Motors GM SL12340 series 6-cylinder gasoline powerplant of 127 horsepower output at 1,800rpm. The powerpack resided in a forward-right hull placement which allowed for a split-door arrangement to be fitted at the rear hull face for the crew exiting and entering the vehicle. Road speeds reached 30 miles per hour with operational ranges at 150 miles though only 47 gallons were held in the internal fuel tank. The engine was mated to an Allison XT-90-2 transmission system and drove the vehicle through a front-mounted drive sprocket. Three road wheels were also in play as was a ground-level track idler at rear of the track system.
The M50A1 variant followed original M50 production marks and these introduced the Chrysler HT-361-318 V-8 gasoline engines which were now coupled to Allison XT-90-5 transmissions. Further differentiating details included louvers added over the hull engine for air intake purposes. Access panels were added at the transmission compartment for ease of maintenance. The new engine increased output power to 180 horsepower. The designation served as the new M50 standard though only 176 of the original stock were upgraded to this from the period spanning 1963 to 1965.
Despite its design as a tank destroyer, the Ontos served a better role as an anti-personnel weapon delivering HE projectiles at range. The North Vietnamese Army did not field a large number of tanks so that limited use of the M50 as a dedicated tank destroyer but excelled its service career as a fire support system. It was not long before the anti-infantry approach was undertaken as a result.
In practice, the Ontos proved that it could be a ferocious weapon under certain battlefield circumstances though it held several major deficiencies as combat vehicles go. Cramped conditions aside, the hull was only marginally armored and its sloped nature only proved effective up to a point, offering little to no protection against hidden mines or rocket grenades. Additionally, the recoilless rifles had to be reloaded by a crewmember from outside of the confines of the vehicle which exposed him to unnecessary dangers both environmentally- and battlefield-related. The internal fuel tank severely limited operating ranges, particularly off road when on uneven terrain.
Despite these limitations, its users certainly seemed to have enjoyed their little "Things". The compact size of the vehicle made them truly portable by several measures including helicopter and rail car. They could traverse softer terrains and confined roads where heavier tanks could not tread. It also held exceptional mobility thanks to its small size and lightweight stature and showcased a relatively low profile for a combat vehicle - difficult to train in on at range by the enemy. In The fire support role, the impressive six rifle collection became a Godsend against enemy infantry under cover to the point that the NVA and Viet Cong forces feared and respected the little American machine.
As with all military vehicles, the end of the line for the Ontos came in 1969 when the active fleet was stood down. From 1970 onwards, the M50s were delivered back stateside and scrapped while a few managed extended lives as museum showpieces.