Unofficial history of the Australian & New Zealand Armed Services 

 Search  &  Help Recruits Military History Hall of Heroes Indigenous Slouch hat + ARMY Today Uniforms Badges

 Colours & Flags Weapons Food Equipment Assorted Medals Armour Navy Air Power 

Nurses - Medical Tributes Poetry - Music Posters & Signs Leaders The Enemy Humour Links Killing Anzac

Subject to Crown copyright. Click to escape.
Category: Armour

Click to go up one level

ONTOS ; the Thing or the Pig

(M50A1) by Michael Scudder

ONTOS (M50A1) (by Michael Scudder)
Ontos means "the thing" in Greek.  As applied to the Marine's armoured vehicle, it could mean "the rare thing". This armoured vehicle made significant contributions to the success of Marine and Army infantry operations in Vietnam, but less is known about the vehicle than any other armoured vehicle produced by the US military.   Even among military vehicle collectors, the name Ontos often draws blank expressions.   
The reasons may stem from the fact that the Ontos was produced in small numbers.  Only 176 vehicles are known to have been in the Marine Corps at the start of the Vietnam War.  Another factor is the Marine Corps quickly disposed of the surplus vehicles; removing much of the hulls and gun mounts.  With so few examples of surviving Ontos making it into the hands of museums and collectors, its story didn't get told.  There are more surviving WWI tanks today than Ontos.

The Ontos was a relatively light weight tracked armoured fighting vehicle that was designed in the early 1950's to destroy the main battle tanks of this era using the firepower from its six 106mm recoilless rifles.  Its diminutive size, 12 ' long, 8 ' wide, crammed three crewmembers into a compartment slightly higher than 4'.  It served the US Marines from 1956 until the bulk of them were dismembered in 1970.  Its service to the Marine coincided with the Corps's use of the 106mm recoilless rife.
The Ontos would be more than 10 years into its life cycle before it would be tested under fire.   The first test would be against the Dominican rebels in April of 1965.   The second test was in the environment of Vietnam; and its role would have no relationship to what was originally intended for this fast little tank killer.
If it is true that an army fights its present war with tactics and equipment from its last war; then it is the mark of a successful army to be able to adapt in order to accomplish the new mission.   The Ontos and its crews had to convince the Marine Corps leadership that this fighting vehicle had a role in Vietnam.  The success, at convincing its leaders of the Ontos's potential, is mixed.  The men that made up the Ontos crews attest that it was only at the company level that they convinced leadership of the enormous firepower that could be available to the grunts; firepower that could change the outcome of a fire fight.
I am struck at the similarities of the Ontos's role within the Infantry Company and the role of the little Stuart tank used by the Marines in the pacific battles of WWII.  Both were lightly armoured and vulnerable to the destruction by weapons above 50-caliber.  Both of these vehicles were effective because they were small yet could carry relatively high firepower into an infantry fire fight.  Their size allowed them to go into areas the larger tanks could not.  
The 20" wide tracks of the 9-ton Ontos would allow it to go on the soft soils surrounding the rice paddies of Vietnam.  They both served as bunker busters.  Both vehicles lessened the infantry's causalities by being close to the fight; and could be quickly deployed to overcome an enemy's fixed positions.
The Ontos carried the beehive round that sent out a hundred darts per firing to clean out a jungle of its enemy.  There was no other weapon that could clear a jungle for a depth of a mile like the 106mm recoilless rifle using the beehive round.  Artillery shells and bombs effectiveness was cut to the area of a direct hit.  
The jungle vegetation absorbed both concussion and fragmentation.  The other vehicles that mounted the 106mm recoilless rifle were open to enemy small arms fire.  The Ontos could expose itself to enemy small arms for the short time it took to empty its 6 guns and depart to a more secure position to reload.  It was an armoured shotgun and the North Vietnamese Army feared it.
It is no surprise to the veteran of any country's army that weapon systems get misused, unsupplied and/or forgotten by the generals that demanded their development.   The Ontos fell into this grouping.  Deployment of the Ontos seemed like an after thought to many commanders and the Ontos's parts replacement was a serious concern.  It is a testament to the men that manned this small armoured fighting vehicle that some important history was written by its participation in Vietnam.
The Ontos was designed in another era for another purpose.  Developed to kill tanks; the Ontos found itself outmoded before it was in the hands of its first crewmen.  It was left to the men who manned the Ontos to reinvent it; and they reinvented it into a weapon that served the Marine infantryman.
The Ontos crews were pulled from the Marine Infantry Battalions to learn the trades of gunners, radiomen, mechanics and tacticians.   The Marine designations for jobs within the infantry battalion were in the series 0300.  The Ontos's crews carried variations on these job numbers.  Some crewmen were motor transport or track maintenance trained, but most were more likely to be former riflemen.  After their tours with the anti-tank units, they were just as likely to return into the battalions from which they came as to be reassigned another anti-tank unit.
The Marine high command was single minded in pitting the Marine Infantryman against the North Vietnamese Army and Viet Cong veterans.   They felt, with good assurance, that the individual Marine, coupled with traditional artillery and air cover, could defeat this enemy without any distractions from those weapons not carried to the field by their enemy.
Everyone believed that Vietnam was not conducive for tank operations.  If it were, the NVA would have fielded armour.  Since tanks were not fielded by the NVA, then there was little use for the small anti-tank known as the Ontos also known as a "pig".
The Marines carried its' M48A2 tanks and Ontos into the fields of battle, but they didn't command the focus of the Regimental and Battalion Commanders as did the traditional attachments such as the 81 and 4.2 mortars, 105mm artillery and close air support. The Ontos's deployment was often a knee jerk decision by Battalion Commanders.  They were used mostly as perimeter defence with some convoy duty.
Vietnam was run by our politicians; with rules of engagement that totally distracted the military commander.  Our air power was forbidden to knock out the surface to air missile sites that depleted their ranks.  In early Vietnam, the enemy could retreat to areas forbidden to US forces.  These rules as applied to the Ontos crews decreed that all major calibre weapons had to secure Battalion authority before being loaded or fired.    The early Ontos crews were expected to go into combat areas unloaded.  Later, they could have 106 rounds in the guns, but had to secure authority to fire. 
These rules would have given nightmares to WWII or Korean War veterans. If the Marine Commanders ignored the attributes of the Ontos, the NVA didn't.  In almost all my interviews with Ontos Crewmen, one point was brought up.   The NVA was frightened of the Ontos and would avoid contact if possible.  Most contact between NVA and the Ontos was inadvertent on the part of the enemy. 
The Ontos project was awarded to the Allis-Chalmers' farm machinery Division of Wisconsin around the early part of November 1950.  The Allis-Chalmers engineering section was comprised of about 50 to 60 engineers; 90% of which would eventually work on the development of the first prototypes.  All the prototypes were constructed in the Agricultural Assembly Plant in LaPort, Indiana.
The project was first envisioned by the government to be an air transportable tank destroyer capable of being lifted by the cargo aircraft of the 1950's.  The contract was to be for 1000 vehicles to be delivered to the Army.    In 1953, the army would refuse to accept delivery of the Ontos.  At this point the Marines accepted delivery of about 300 vehicles.
The government Ordinance Command, represented by Chief Engineer Carl Holmyard, delivered only one page of specifications.  The specifications demanded that the vehicle would be powered by the same GMC six cylinder gas engine that was standard for the 2 ton military trucks of the day and a front mounted Allison cross drive transmission that would carry power to the tracks.   The remainder of the specifications restricted the outside dimensions and weight so as to be air transportable.
The project was classified "Confidential".  
This is the lowest security classification for government work, but it still required that the prototypes be built in a walled off section of the Agricultural Division Assembly Area.  The government would accept the prototypes for testing only after the machines had 50 hours of running time.  This required the engineering section to come to the plant on weekends and drive the prototypes around the grounds of Allis-Chalmers.
The Ontos had two large arms that held the six recoilless rifles.  These arms were joined to a shallow turret.  This entire assembly was cast in armoured steel.   The early prototypes could swing the guns less than 15 degrees left and right.  The production Ontos was modified to turn the guns 40 degrees left and right. The welding of the Armoured hull was a problem for Allis-Chalmers.  It took the failures of several prototypes to develop the proper welding techniques.
The first prototype Ontos had a track system similar to that used on the self propelled artillery vehicle called the Scorpion.  This track system was later changed.  This first Ontos prototype is still in existence and in the hands of Mr. Fred Ropkey. The later and final track system and suspension was of a new design.  Each track consisted of two sections of rubber; 48" long with steel drive teeth in the centre.  Twenty-inch wide steel grousers held the rubber and the drive teeth together.  It took 5 sections of track to make up a complete track.  A well-motivated crew could accomplish a track section replacement in about 1 hours.  One crewman told me that a crew made a track repair in 42 minuets.
The suspension system was designed so that no mechanisms intruded into the already small interior of the fighting compartment.  The road wheels hung on torsion bushings that were attached to the sides of the hull.  There was much development in the special rubber compounds for the bushings.
The rubber bladder gas tank was mounted in the front of the vehicle directly behind the glacis plate.  It was cast of rubber and contained a tube-shaped void through its centre to allow the left drive shaft to pass through the fuel tank on its way to the left drive sprocket.
Allis-Chalmers had developed a deep water fording provision for the Ontos that was not accepted by the Marines.  It consisted of a waterproof covering for the engine.   The motor would stay dry while fording.  The fording gear had to be carried and installed on the Ontos prior to fording.
One of the problems that had to be overcome by Allis-Chalmers involved the track alignment.  The lower chassis was constructed as a weldment.  The distortion involved in the welding process caused the suspension to become out-of-line; and so the track would be thrown.  Machining the lower hull, where the track suspension parts bolted to the hull, finally solved the problem.
Allis-Chalmers also developed a personnel carrier based on the Ontos track design.  The personnel carrier had one additional 48" track section.  No photos are known to exist of the personnel carrier prototype.
Much of the engineering work was completed in 1950 during a two-week design marathon.  The Marine Corps continued to test the vehicles for the next six years until the vehicle was accepted in late 1956.  A review of the chief engineer's notes for 1957 through 1959 showed a continuing series of revisions.  
These notes, of Chief Engineer Craig Cannon, referred to a major revision of the Ontos called the "1960 project".   Some of the proposed revisions called for an aluminium amphibious hull and two 105mm recoilless rifles (designated as T237 guns) fitted with a cylinder similar to a revolver pistol.  This change would have allowed multiple firings of the two guns without the need for a crew member to reload the guns from outside the vehicle. Another major revision would involve the replacement of the engine with a turbine engine.  
The "1960 Project" was never accepted. One of the early tests involved the acceptance of the aiming system of the six gun turret.  Part of the test included the firing of all six guns at once.  The test vehicle was taken to the Aberdeen testing facility that had been built for the testing of the 106 recoilless rifle.  No one envisioned the effect of six of these weapons going off at once, least of all the people who designed the testing facility.  The back blast from the firing knocked bricks out of a nearby building and knocked the rear windows out of several cars.
Allis-Chalmers was to later refurbish the Ontos: removing the 6 cylinder engines and replacing them with the 361 cubic inch Chrysler V8.  The change over involved redesigning the armoured engine covers with additional venting.  It is believed that, of the 300 delivered units, only 176 Ontos were refitted.
The 51"wide glacis plate of the Ontos is 1" thick.  The glacis plate forms the front of the hull and would protect the driver and transmission from ground level to 27"in height.  The side plates that hold the track suspension parts and form the sides of the crew compartment are slightly heavier than 1/2" thick.  The floor of the fighting compartment is " thick of non armoured steel.  The majority of the remainder of the hull is formed from thick armour.  The front engine covers are cast of armoured steel and its louvers have a 3/8' bead formed on the inside lip of each louver to defeat the entry of small arms fire from entering the engine compartment.
It has been speculated that the Ontos was top heavy and tended to overturn easily.  I found that with the top hull, gun mount and guns removed, the vehicle still weighed in at more than 11,000 pounds. This 11,000 pounds would be contained within the height of the tracks (34").  I therefore doubt that the Ontos was seriously top heavy.  Crewmembers have told me that the vehicle would slip sideways if traversing a steep hill before it would roll over.

The 106MM Recoilless Rifle Firing System
The Ontos had the ability to fire its 106 recoilless rifles one at a time or as many as all six guns at once.   Four of the six guns had 50 calibre spotting rifles attached.  The flight of the 50 calibre spotting round approximated the flight of the 106 round.  This round was constructed as a tracer with a smoke puff that appeared on impact.  The firing of the weapons was directed by the gunner; who had a seat to the rear of the driver and engine.  
The gunner would often first fire the spotting round at the desired target and watch its flight.  Often, even prior to the spotting round hitting the target, the 106 round would be sent on its way.  The maximum effective range for the 106 round was 3,000 yards.  The 106-MM rifle was generally considered a direct fire weapon, but the crews were taught, and used, indirect fire at targets not within sight of the gunner. 
The 106-MM recoilless rife is more than 11' long and weighs about 288 pounds each.  The turret of the Ontos had to carry this 1,700 plus pounds over uneven ground.  The strain on the gun mount required the crews to realign the guns from time to time.
Two of the six rifles were designed to be easily removed from the vehicle and used with a ground mount should it be required.

There were three men to an Ontos: driver, commander/gunner and loader.  If the Ontos was carrying a lot of ammunition and/or other gear, or if the weather was extremely hot, you could find the loader sitting on the driver's hatch, riding on the machine gun crossbar or riding in the platoon's ton Dodge truck that often escorted a platoon movement.  
The Ontos could and did drive with the rear doors open on occasions.  This mode of travel would roll road dust into the interior making the crew look like pigs.  For this reason the crews usually referred to the Ontos as a "pig".
The training of the crew varied as the demands of the war changed.  The early crews, prior to March 1965, were trained at Camp Horno in the home of the 1st Marine Division, Camp Pendelton on the coast of California near Los Angeles.   The training of later crews were assigned to units that trained them in the field.  They had to learn: vehicle maintenance, small arms, tactics and direct and indirect fire.   
All Marines were trained in small arms, but the Ontos crews also carried a sub-machine gun that was not used by most Marine units.  They also had to know how to operate the three main radios and intercom.  Some of the Vietnam assigned crews went to the firing ranges on Okinawa for extensive day and night-time firing of the 106MM rifles.
The Ontos crews were required to replace the 48" long sections that made up the track.  They also tightened the track adjustment when a track was repaired or when a series of hard turns stretched it.  An Ontos mechanic was assigned to each platoon, but the crewmen had to assist to keep the machines running.  The platoons were often separated when assigned to infantry units.   
The platoon mechanic was often unavailable to make a repair when needed.  Replacement parts were often rare or non-existent.  Many Ontos were turned into parts vehicles due to poor parts supply.
The Ontos platoons were organized into heavy and light sections.  There were three Ontos in a heavy section and two Ontos in a light section.  There were three platoons to a company; and three Companies to an Ontos Battalion.  The 1st and 3rd Ontos Battalions saw action in Vietnam.  The machines were in Vietnam from early 1965 to mid 1969.   There is some evidence that at the end of 1965 there were 65 Ontos in Vietnam.  
If this figure is correct it may have represented 45 Ontos of the 3rd Marine Anti-Tanks and a lesser number from the 1st Marine Anti-Tanks; as some of the machines were aboard ships in a standby mode awaiting to be deployed in any hot spots that arose.
The Ontos crew carried 6 of the 106MM rounds in their guns.  They carried 8 shells in the rear storage area under the rear doors and 4 rounds in a rack located in the right rear of the vehicle.  The loader would dismount and reload from this ammo locker.  The interior of the vehicle may carry additional shells depending on the situation.  I interviewed an Ontos platoon Sgt. that removed both the driver's seat and commander's seat and piled 30 additional shells into the cramped space.  He sat on the ammo while driving or firing the weapons.
The crew also carried an M-3A1 submachine gun (also known as the grease gun or SMG) and Colt 45 automatic pistol (worn in a shoulder holster) with 250 rounds, 1,000 tracer rounds for the 50 calibre M8 spotting rifles, and 1,000 or more rounds for the 1919A4 Browning machine gun.  Many of the crewmen carried personal weapons.  
Some of the personal weapons included shotguns and captured weapons such as the Thompson submachine gun, AK-47 and SKS communist made rifles as well as the French made submachine guns.  One crew, known to me, also mounted a 60MM mortar on the front plates of the Ontos.
The Marine Infantry Battalions were armed with the M60 machine gun during this period.  For some unknown reason, the Ontos continued to carry the older light Browning machine gun.  Some of the crews had the option of changing to the more modern, fully automatic weapon, but opted for the Browning. The older Browning had a reputation to require fewer barrel changes from heavy use.  The Browning machine gun was mounted on a pipe support attached to the gun mount/turret.  It could be fired manually or remotely from inside the Ontos by way of a foot pedal.

What happened to the Ontos?
The Marines deactivated the Ontos from Vietnam in May 1969.   A few of the Ontos were left in Vietnam and turned over to an Army Light Infantry Brigade near Tam KY.   The Army ran them until they ran out of replacement parts.  They then made them into fixed bunkers.  The remainder of the machines in Vietnam were loaded into ships in May of 1969 for return to the US.  The crews were reassigned to various Marine Infantry Battalions.
Once the machines were returned to the US, their top hulls were cut off and many of the chassis were sold for construction equipment or given to local governmental agencies for rescue work.

Statistics : Over 35 million page visitors since  11 Nov 2002  



 Search   Help     Guestbook   Get Updates   Last Post    The Ode      FAQ     Digger Forum

Click for news

Digger History:  an unofficial history of the Australian & New Zealand Armed Forces