The odd-looking Ontos was originally intended to be a nimble, light-wight, and easily transportable tank destroyer platform that could be rushed around battlefields by air. While the Ontos was indeed a powerful and relatively lightweight platform, its intended role as a tank destroyer was not exactly useful during the conflict in Vietnam thanks to the very modest amount of armored vehicles fielded by North Vietnam. In lieu of armored threats, U.S. Marines adjusted the M50’s mission profile from anti-armor to anti-personnel. In this role, the Ontos excelled.
Sporting a total of six M40 recoilless rifles on two arms at either side of the turret, the M50 Ontos could unleash an incredible amount of firepower at once. When used against enemy defensive positions, particularly in buildings or other reinforced positions, the Ontos was very effective. And, in addition to high-explosive anti-tank rounds, the M50’s recoilless rifles could be loaded with anti-personnel canister ammunition filled with metallic flechettes.
Like individual M40 rifles, the Ontos’ rifles were zeroed in using a .50 caliber spotting rifle. The high-caliber round was not used for direct combat itself. Instead, its trajectory was closely matched to the M40 rifle it was mated to. Where the spotting rifle's bullet landed, so too would M40 rounds.
Thanks to the Ontos’ light sub-ten ton weight and relatively wide tread design, the tank killer enjoyed good mobility, particularly in the wet and mushy conditions often seen in parts of Vietnam.
The M50 Ontos was a powerful psychological weapon as well thanks to its distinct and easily-identifiable profile. Wartime accounts by veteran Marines from that conflict suggest that the M50’s destructive potential was recognized by North Vietnamese and actively avoided.
Good, Not Great
Still, despite the Ontos’ not inconsiderable firepower, it was far from a perfect anti-personnel platform. Yes, it could fire six rounds of 105mm HEAT or flechette simultaneously, but the M50’s six rifle tubes had to be loaded from the outside the vehicle hull, dangerously exposing the tanker crew.
The Ontos also suffered from a modest armor package. Though adequate to protect from most small-arms fire, the M50 was vulnerable to anti-tank weapons like mines. In addition, the Ontos had a very tall profile for such a small vehicle and therefore struggled to find adequate cover in some situations, leaving the M50’s driver, gunner, and loader dangerously exposed.
By the late 1960s, most if not all of the M50 Ontos in Marine Corps service had been deactivated and either handed over to the Army, scrapped, or converted for other civilian purposes as heavy construction movers. Though relatively short-lived, the M50 made a big impression: if you see six recoilless rifle barrels pointed at you, get out of there as quickly as possible.
Caleb Larson is a defense writer for the National Interest. He holds a Master of Public Policy and covers U.S. and Russian security, European defense issues, and German politics and culture.