By any measure, the Ontos was one of the most interesting things to come down the road of United States military armored development. The idea for this vehicle was born in the aftermath of World War II when the U.S. Army perceived the need for a new reconnaissance vehicle. Then it evolved into a tank destroyer for use with the Army on the nuclear battlefields of Europe. Next it was deployed in Marine Corps anti-tank battalions. The Ontos most significant contribution was in the Vietnam war, but in a role much different than the role for which it was designed. This is the story of the Ontos, officially the "Rifle, Multiple, 106mm, Self-Propelled M50 of the antitank company, infantry regiment, Marine Division."

The adaptation of the internal combustion engine to warfare brought about the removal of the horse from the battlefield. The reconnaissance mission formerly performed by cavalry remained. By the end of World War II, the motorcycle, jeep, armored car, and light tank all tried to fill the gap, all without complete success. A classified 1953 U.S. Army report noted: 

There is an urgent and immediate need in our army for a vehicle similar in performance to the jeep, but at the same time affording some armor protection and greater cross-country mobility, for use by reconnaissance personnel, commanders, messengers, and liaison officers who are frequently exposed to small arms fire.[1]  

At that time jeeps and half-tracks were authorized in the command, scout, and support elements of the Army reconnaissance platoon. The Ontos was considered as a replacement. After considerable study the Army concluded that although the vehicle had outstanding cross-country mobility and armor protection, it had deficiencies in the areas of storage space, lack of speed, lack of range, and excess weight. Ironically, given the vulnerability of the M50 to enemy mines in Vietnam, the Army concluded these test vehicles "offered protection against atomic bombing."[2] The Army decided to stick with its M38A1 half-ton trucks and M21 mortar carriers for reconnaissance platoon use.

During World War II, the Army embraced the tank destroyer concept, which called for the placement of large-caliber anti-tank guns on lightly armored carriages. These could quickly be moved to any area under enemy tank threat. This concept was never embraced by the Marine Corps to any extent. The tank remained the favored anti-tank weapon for the Marines in the immediate postwar period. In addition to duties as naval infantry, postwar planners envisioned a role for the Corps in any European conflict between the United States and the Soviet Union. Late 1940s war planning put the Marines into direct conflict with front-line units of the Red Army. In the Pacific War the Marines dealt with sporadic attacks by small Japanese tanks. In the future war Marine tankers would have to face a highly mechanized Soviet force equipped with large numbers of medium and heavy tanks. [3]  

Using tanks to destroy enemy tanks proved less than satisfactory in the Korean War: too often the weight of American medium tanks rendered them too roadbound. Both Army and Marine planners, cognizant of the formidable threat posed by Communist armor, returned to the World War II tank destroyer concept. In 1949 the USMC Armor Policy Board specifically noted "There is a requirement for a destroyer type tank to destroy hostile heavily armored vehicles . . . ."

 As early as 1944 Army production and logistics considerations began to determine Marine Corps tank decisions.  Although some of the USMC armor requirement was developed and produced by the Navy's Bureau of Ships (e.g., amphibious tractors or amtracs), the Corps came to fully depend on the Army for its tank procurement. [4] In 1951, Marine Corps planners and engineers from the Allis-Chalmers Corporation began development of this new anti-tank vehicle. It would be built at the company's La Porte, Indiana, factory. [5]

  In 1953, Michigan Congressman Gerald R. Ford held congressional hearings for Army appropriations. When discussion turned to anti-tank capabilities, the testimony of Army generals was taken off the record and not included in the printed transcript. [6] The public became aware of Ontos development only by mistake. According to a report in the New York Times dated June 26, 1953, the congressional testimony was classified secret. The newspaper noted "An entirely new weapons-carrying vehicle, nicknamed 'The Thing' but carrying the official designation Ontos, to be used variously, including as a mount for a new 'very high-powered' recoilless rifle and for a quadruple .50 calibre antiaircraft weapon against low flying planes.' Army officials expressed amazement and appeared appalled when copies of the 1,667-page printed testimony released by the subcommittee reached the Pentagon.

The original M-50 Ontos emphasized firepower over crew comfort. The hull was derived from the T55/T56 series of tracked armored personnel carriers. It was powered by a six cylinder in-line gasoline engine, the General Motors SL 12340, which developed 145 horsepower at 3,400 rpm (this was later upgraded to a Chrysler V-8). This power source was coupled to a XT-90-2 transmission, which drove the front sprockets, which turned the tracks. Maximum road speed was 30 mph on improved roads. The Ontos had terrain navigation ability superior to tanks. Range was 190 miles on primary roads, 120 miles on secondary roads, and 50 miles cross country with a 47 gallon internal fuel tank. With fording kit installed the vehicle could cross streams as deep as 60 inches. This three-man vehicle weighed nine tons. It was not portable by any available helicopter although it could be air transported by R4Q aircraft. Two Ontos could be landed over the beach in a LCM-6 (Landing Craft, Mechanized).

The main weapon consisted of six 106mm M40A1C recoilless rifles mounted on a central turret that overhung the hull on both sides. Built with simplicity in mind, this rifle was the same weapon used by infantry on a fixed mount. These guns could be fired individually, in pairs, or all at once. Fifty caliber spotting rifles were mounted on four of the recoilless rifles. Two of the recoilless rifles were equipped with a spotting rifle and sight and could be removed from the vehicle for use on ground mounts (these spotting rifles could not be fired from inside). The Ontos also had a .30 caliber machine gun. These weapons were externally and coaxially mounted and were fired electrically. Rate of fire was four aimed rounds per minute with all guns loaded. Average reload time was one minute.

The trajectories of the spotting rounds and the 106mm rounds were very similar to a distance of 1,100 yards. Beyond 1,100 yards the trajectories differed, causing the effectiveness of this spotting system to decrease as range increased. The spotting rifle could not be used beyond 1,500 yards, necessitating the use of burst-on-target and bracketing techniques of fire adjustment at these greater ranges. High Explosive Anti-Tank (HEAT) ammunition for the 106mm rifle would penetrate the armor of any known tank (16" of armor at 0 degrees obliquity). The Ontos' armor was one-half inch plate except for the floor, which was only 3/16" thick. The upper sloped armor would withstand all small arms fire, but was vulnerable to .50 caliber armor piercing rounds.  Artillery airbursts could cause severe damage to the Ontos' guns and external fire control equipment.[7]

Frank Pace, Jr., Secretary of the Army during the Truman administration, initially supported the M50 for Army use. Pace noted, "if Ontos is there, tanks had better get the hell off the battlefield."[8] Not everyone agreed with Pace. Others felt it was too lightly armored, underpowered, and incapable of sustained combat. The Marine Corps accepted the Ontos after the Army rejected it. The Marines did not have the specialized supply and maintainence capabilities of the Army, and Ontos was a simple vehicle. It had fewer parts than other armored vehicles. There was no heavy turret. The engine was a common truck engine found on various military and civilian vehicles. The fire control system was simple: according to Lieutenant Colonel E. L. Bale, Jr., a Marine instructor at the Army Armor School, the average Marine could master the system "as easily as he has the pinball machine in the local drug store."[9] The Corps ordered 13 million dollars worth, about 300 vehicles. Production was to run for about one year beginning in mid-1956.

The Ontos was quickly integrated into armored units of the Fleet Marine Force. Its first non-training deployment abroad came in July 1958. The Lebanon Crisis saw Marine Battalion Landing Teams (BLT) of the Navy's Sixth Fleet come ashore to stabilize the weak Lebanese national government. This 2d Provisional Marine Force included 15 M48 tanks, ten Ontos, and 31 LVTP5 amtracks (Landing Vehicle, Tracked). These vehicles provided general Force security and protection for armored patrols until a larger Army tank force could be sent from Germany. The Marines began reembarkation in mid-August.[10]

Marines got their chance to destroy tanks with Ontos in 1965 in the Caribbean. In April the Dominican Republic was sliding into civil war as reformers did battle with right-wing military forces. The 6^th Marine Expeditionary Unit, the Caribbean quick-reaction force, was ordered to ashore in order to evacuate civilians and reinforce security at the American embassy. A full company each of tanks, amtracs (LVTs), and Ontos were part of the landing team. The U.S. force took the side of the Dominican military. The rebel army posessed less than formidible armor capabilities. Before leaving in June, M50 HEAT rounds scored hits on an old Swedish Landsverk L-60 light tank and a French AMX-13 light tank.[11] But it was halfway around the world in Vietnam where the Ontos mettle would be truly tested.

 Ontos were organized in the Marine Division into Anti-Tank Battalions. Each battalion was composed of one headquarters and service company plus three anti-tank companies. Each of these latter companies contained three platoons of five Ontos for a total of 45 Ontos vehicles per battalion. Planned distribution in the Marine Division was for 15 Ontos (three platoons) for each of the division's three infantry regiments. Ontos companies, along with tanks and amtracs, landed with the 9^th Marine Expeditionary Brigade at Danang, Vietnam, in the first half of 1965. Within one year, both the 1^st and 3^rd Anti-Tank Battalions were ashore in Vietnam. Unlike the enemy in the Korean War, the Vietnamese Communist military forces possessed significant anti-armor capabilities in the form of recoilless rifles and rocket propelled grenades. The Ontos' thin floor armor (3/16") made it especially vulnerable to mines. Consequently, and as opposed to its designed role, Ontos spent a great portion of their time in static defense positions.

 Initially Ontos units were deployed in defense of the Danang airfield. In August, 1965, the Marines began Operation Starlite, the first big battle of the war. At 0730 on August 17, tanks and Ontos rolled off amphibious landing craft (LCUs and LCMs) and made their way ashore south of Chu Lai in support of the assault companies. Later in the day a Marine armored column was halted when a M-48 tank was hit with recoilless rifle fire. The Viet Cong (VC) poured mortar and small arms fire into the Marine positions, quickly killing five and wounding 17. The Ontos manuvered to provide frontal fire and flank protection until enemy fire let up. The following month, in Operation Golden Fleece, a combined infantry-armor assault force including Ontos attacked a VC main force unit trying to collect a rice tax in a Vietnamese village near Danang. The enemy was forced to break contact and flee the area.[12] 

After establishing themselves at Danang and Chu Lai, the Marines built their third base at Phu Bai, in Thua Thien Province 35 miles northwest of Danang. Initially, defense at Phu Bai was provided by the 2d Battalion, 1^st Marines (Reinforced) which had a platoon of M50s attached. It was not only the Marines who were expanding their forces in the northern part of South Vietnam: both the VC and North Vietnamese Army (NVA) also increased their forces, and both sides sustained heavy casualties. In late June, on Operation Jay, a large, heavily armed VC force ambushed a South Vietnamese Marine Corps convoy moving north on Route 1, the main north-south highway in Vietnam. At 0830 hours on June 29, the attacking force struck the convoy with mortar and recoilless rifle fire, quickly hitting ten trucks. U.S. Marines quickly sent reinforcements, including Ontos, to assist the SVN Marines. The VC force lost interest and tried to break contact. While crossing open ground, the M50 platoon opened fire and "obliterated a VC squad on a ridgeline with a single 106mm salvo." A M50 platoon commander even captured an enemy soldier. Over 185 enemy soldiers were killed in this action.[13]

Marines and their armor were deployed in I Corps, the northernmost of four military districts in Vietnam. An exception to this was Special Landing Force (SLF) of the Navy's Seventh Fleet, the strategic reserve for the Pacific Far East. The SLF was available for amphibious landings in South Vietnam. General William Westmoreland, commander of U.S. forces in Vietnam, decided to use the SLF to clear Viet Cong forces from the Rung Sat Special Zone south of Saigon. VC gunners were firing on ships using the river channel that supplied the Vietnamese capital. The result was Operation Jackstay, March 26 - April 6, 1966. The operation had limited success but not due to lack of ingenuity of the Marines, who experimented with riverine warfare techniques including mounting an Ontos on a LCM to provide fire support. Only 63 enemy were killed; however, the shipping channel was at least temporarily clear.[14]

The following incident illustrates the vulnerability of the M50 to enemy mines. It was spring, 1966. An armored column supporting Company "K", 3/9 was returning to base camp near Danang. Three tanks and an Ontos went over a stream at a place called the Viem Dong Crossing without mishap. As the second M50 crossed, Platoon Commander Allen Hoof heard a "pop," turned rearward, and saw the upper half of the vehicle blown off the lower half, and lying upside down next to it. All three crewmen were wounded. Acting Ontos Commander PFC Greg Weaver was quickly removed from the vehicle but died almost immediately. The mine explosion, perhaps either command detonated or activated by a counter, caused the detonation of a 106mm round stored directly under  the commander. This secondary explosion blew the turret off the vehicle and killed Weaver.[15]

Since enemy tanks were not a problem for Marines in Vietnam, Ontos use reverted to its secondary mission:  providing direct fire support for infantry. By late 1966 problems with Ontos became evident. The supply of tracks was depleted, which caused breakdowns on operations. This caused a reluctance to utilize the M50. An even more important reason was several incidents of accidental firings of recoilless rifles which cost some Marine lives. This was an extremely serious problem for Ontos on convoy duty. These accidents were caused by overly tight adjustment of the firing cable allowing the firing pin to release prematurely. This adjustment was a crew responsibility and required a thorough understanding of the firing cable, sear, and trigger. These mishaps caused restrictions to be placed on Ontos' use.[16]

By 1967 the Marines were fighting two wars in Vietnam. The 1^st Marine Division engaged in counterguerrilla operations in the southern part of I Corps while the 3d Marine Division conducted mostly conventional war against NVA along the Demilitarized Zone (DMZ) in the north. As the Marines moved northeard to counter the NVA threat, Ontos and tanks provided important support. In May 1967, the 2d Battalion, 9^th Marines (2/9) and 2d Battalion, 26^th Marines (2/26) began Operation Hickory north of Con Thien. Fighting against enemy forces in well prepared bunkers and trenches was heavy. M50s, using the proper ammunition, proved to be devastating antipersonnel weapons. After the conclusion of Hickory, 2/9, accompanied by tanks and Ontos, was sent on a spoiling attack into the DMZ. On this operation the tracked vehicles proved more of a liability than a tactical asset as the terrain restricted them to the road. Instead of providing infantry support, the M50s and tanks required infantry protection against NVA rocket propelled grenade (RPG) attack. Using these vehicles as ambulances to evacuate the wounded further reduced their offensive capabilities.[17]

 1967 saw the introduction of CH-53 Sea Stallion heavy-lift helicopters for the Marines in Vietnam. The first models had a six-ton external lift capability. This meant an Ontos could be transported by helicopter if it was broken down into components with the hull transported externally. It could then be reassembled and operated at destination, given it a transportability beyond its design considerations. M50s could also go where tanks feared to tread (or should have): in a 1966 operation, tanks got stuck in flooded rice paddies. Ontos, with less ground pressure, were able to drag timbers up to the tanks without bogging down. In Operation Jay, mentioned above, The Ontos of B Company, 3^rd Anti-Tanks were able to assist the SVN Marines because they could cross a pontoon bridge -- the only tracked vehicles light enough to drive to the operation. Ontos could go more places than many people thought possible.[18]

 In December 1967, the 1^st and 3d Anti-Tank Battalions were decommissioned in Vietnam and their vehicles were attached to the tank battalions.[19] 1968 saw Ontos assume an important role in some of the heaviest fighting of the entire war, the Battle of Hue during the Tet Offensive. In February, 14 NVA battalions seized control of most of the city. The Americans and South Vietnamese faced the formidable task of retaking  this important cultural center of the nation. The result was urban fighting unlike anything seen in the war. The attacking Marines had to take each building and each block one at a time. This close-quarter combat and low-flying clouds, coupled the desire to minimize damage to the city itself, meant there could be little reliance on artillery and close air support. 

Four tanks from the 3d Tank Battalion along with a platoon of Ontos from the Anti-Tank Company, 1^st Tank Battalion, joined the advance against strong enemy resistance. Lieutenant Colonel Ernest Cheatham, comander of 2/5, had reservations about using tanks. One tank sustained over 120 hits and another went through five or six crews. Infantry commanders liked the Ontos better. Cheatham described the M50 "as big a help as any item of gear we had that was not organic" to the battalion. Regimental commander Colonel Stanley Hughes went even further when he claimed the Ontos was the most effective of all the supporting arms the Marines had at their disposal. Its mobility made up for its lack of armor protection, noting that at ranges of 300 to 500 yards, its recoilless rifles routinely opened "4 square meter holes or completely knock[ed] out an exterior wall." The armor plating of the M50 was sufficient protection against enemy small arms fire and grenades. However, B-40 anti-tank rockets were a different story: an Ontos 1/1 was knocked out and the driver killed on February 7 while supporting the 1^st Battalion, 1^st Marines.[20] The essential role of tanks and M50s in the fighting is illustrated by the fact that Marines had to hold up their advance from time to time for lack of 90mm tank and 106mm Ontos ammunition.[21] 

The Perfume River flows through Hue. After clearing the south bank on February 11, the 1^st Battalion, 5^th Marines pushed north to clear NVA forces firmly entrenched in the 4-square-mile Citadel, location of the former Imperial Palace. USMC M-48 tanks and Ontos were placed under the command of the attached tank platoon commander. Tactically, the tank or Ontos commander, working with the infantry commander, would reconnoiter a particular target area, usually a masonry structure blocking the Marine advance. Returning to their vehicle, the tank or Ontos commander return to his vehicle, then move forward at full speed while the infantry laid down a heavy volume of fire. Upon reaching a position where fire could be directed on the target, the vehicle commander halted the vehicle, fired two or three rounds into the structure, then quickly reversed direction and returned to friendly front lines. Casualties among armor crews were high. On February 24, South Vietnamese troops finally dislodged NVA forces from the Citadel. By the time the battle for Hue was over, fifty percent of the city was destroyed.[22]

Before, during, and after the Battle of Hue, the 26^th Marine Regiment was fighting the North Vietnamese at Khe Sanh. Here the enemy tank threat was real: 17 days into the battle at Khe Sanh, NVA tanks helped overrun the nearby Special Forces camp at Lang Vei. Ten M50s from the 3d Tank Battalion were incorporated into Khe Sanh's defenses. They were sometimes used for reconnaissance but more often in static perimeter defense roles. Author Robert Pisor notes the Ontos at Khe Sanh had "enough flechette [anti-personnel] ammunition to pin the entire North Vietnamese Army to the face of Co Roc Mountain."[23]

The Marine Corps began to deploy its forces out of Vietnam in 1969. Tank and amtrac units rotated early as fighting had ebbed in the Corps' area of responsibility. By this time the M50 parts supply was depleted and the 106mm rifle was about to be replaced by other weapons. M50 mechanics cannibalized disabled machines to keep others running, but after Hue the Ontos were worn out. Ironically, excess Ontos were given to Army forces (recall that the Army initially rejected the Ontos as being unsuitable for its requirements). These Army Ontos were formed into Company D, 16^th Armor, for use with the 173^rd Airborne Brigade. The Army used its Ontos until they ran out of spare parts, then employed them in fixed bunkers. In the United States, the Marine 2d Anti-Tank Battalion was disbanded along with the 5^th Marine Division. The last Ontos garrison was stationed at Guantanamo Bay, Cuba. It continued to operate until 1980 by which time it had one operational vehicle. Two others were used for parts.[24]

Upon return to the United States, the tops of the vehicles were removed. Many of the chassis were sold for use as construction equipment or given to local governments for rescue work. One "platoon" of surplus M50s wound up in the service of the North Carolina Forestry Service for use as fire fighting vehicles. According to Vietnam veteran and former Marine Mike Scudder, Ontos today are scarce. In fact, there are more surviving World War I tanks than Ontos. Scudder should know: he bought the seven from North Carolina and is restoring two of them.

Was the Ontos a successful addition to the Marine Corps arsenal? The answer is quite simply, yes and no. The primary mission of the M50 was a tank destroyer. In the actual tactical environments in which it was deployed, there was little use for this ability.

Its secondary mission was the provision of direct fire support for the infantry. In this role the Ontos was underutilized. The reason, according to Major D. C. Satcher writing in  The Marine Corps Gazette, is because, unlike artillery, air, and tanks, Ontos were little emphasized in Marine officer training. Ontos were never used in any tactics problems in The Basic School. Ontos crew did not have their own MOS. An Ontos officer normally served one tour with an M50 unit, then moved on. A weapons system that is under-emphasized will be underutilized.[25]

Rejected in the beginning by the Army as a reconnaissance vehicle, it was used in this role by the Marines in Vietnam. The M50 could go places no other Marine armored vehicle could go. It had limitations. In addition to the problems previously noted (premature firing and vulnerability to mining), the recoilless rifles had to be loaded externally which meant the crew had to leave the protection of the armored hull in order to reload. The 106mm recoilless rifle is no stealth weapon: when fired, the tremendous backblast makes the Ontos' location visible to the enemy. Ontos crew had to ensure no friendly troops were in the large blackblast area when operating in confined areas.

There was no enemy armor for the Ontos to destroy in Vietnam. Still, Marines are famous for their ability to improvise, and enemy infantry were plentiful. The M50 was a formidible anti-personnel weapon. A couple of Ontos on the perimeter could decimate Communist forces attacking Marine fixed positions, a static role quite the opposite of its designed high-mobility anti-armor role.  My favorite example of Marines ability to adapt to local tactical conditions is the mean streets of Hue City in February 1968. Not only good at destroying structures, Ontos were able to provide a "smoke screen" for infantry attacks: when phosphorus rounds were unavailable, "beehive" rounds (explosive shells filled with thousands of small darts) fired into masonry structures created a dust cloud that screened infantry movement.[26]  Marine infantry loved their Ontos. In Phase Line Green : The Battle for Hue, 1968, author Nicholas Warr describes how the M50 platoon pounded the enemy positions, accompanied to the choruses of "Get some!" sung by infantry holed up in houses, waiting to move forward. Fact is, even with its limitations, the Ontos was used and, to a considerable degree, used up in Vietnam, providing invaluable support for the Marines in I Corps.

 For additional reading:

Kenneth W. Estes, Marines Under Armor (Annapolis, MD: Naval Institute Press, 2000).

William B. Allmon, "The Ontos," Vietnam, August 1994.