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Length: 12 feet 7 inches
Width: 8 feet 6 inches
Height: 7 feet
Weight: 19,050 pounds
Crew: Three
Armament: Six 106 mm recoilless rifles with 18 HEAT (high-explosive anti-tank), HEP (high-explosive plastic) or antipersonnel rounds; four .50-caliber M8C spotting rifles with 1,000 rounds; one M1919A4 Browning .30-caliber machine gun with 80 rounds
Gun elevation/depression: +20 to -10 degrees
Turret traverse: 40 degrees left or right
Armor: Hull and turret, 1/2 inch; hull floor, 1/4 inch
M50: General Motors 302 inline V6 liquid-cooled engine (145 hp)
     M50A1: Chrysler HT-361-318 V8 liquid-cooled engine (180 hp)
Maximum speed (road):
30 mph
M50: 125 miles
     M50A1: 100 miles

By 1949 the U.S. Army had abandoned its wartime concept of specialized tank destroyers in favor of main battle tanks. After experiencing mobility limitations during the Korean War, however, the Army conceived a new breed of tank destroyer, one that packed tank-busting firepower on a lightweight, airmobile tracked chassis. The resulting M56 Scorpion boasted a 90 mm gun behind an armored blast shield, while the M50 Ontos packed six M40 106 mm recoilless rifles. Neither weapon ever engaged a tank, but both saw plenty of action supporting infantry in Vietnam—the Scorpion with Army airborne forces, the Ontos with the Marines.

Attached to four of the Ontos’ recoilless rifles were M8C .50-caliber spotting rifles, which fired rounds that produced visible puffs of smoke on impact, thus enabling operators to line up the main guns. Although its recoilless rifles had to be reloaded externally, potentially exposing its crew to enemy fire, the two upper ones could be removed and used autonomously. During the 1968 Battle of Hue the Ontos won favor among troops for its speed, agility, relatively low profile and the ability of its concrete-penetrating high-explosive shells to bring down walls. The two Ontos platoons defending the firebase at Khe Sanh usually hid by day and moved into fighting positions at night. During perimeter patrols the M50A1s of Company A, 3rd Anti-Tank Battalion, towed improvised sleds to supply Marines in outlying, exposed positions.

By 1969 the M50 had passed its prime and was gradually withdrawn, its parts and guns salvaged or repurposed. The last operational Ontos was pulled out of Guantanamo Bay, Cuba, in April 1980. MH