Monday 16 May 2022

Halftrack vs Tanks

The Gun Motor Carriage M3 was one of the many specialized variants built on the M3 halftrack APC. It was armed with an obsolete gun and didn't satisfy American requirements for a tank destroyer, but nevertheless this vehicle became the first mass produced American tank destroyer.

The German Blitzkrieg of May 1940 when German tank spearheads cleaved through French defenses like a hot knife through butter shocked the American military. The Americans suddenly realized that they did not have effective anti-tank weapons. Production of the 37 mm M3 gun was just beginning. Only 340 of these guns were built between July of 1940 and the start of 1941. On the other hand, the army had over 4000 75 mm M1897 guns (licensed copies of the French Mle. 1897) The ballistics of these weapons allowed them to combat all modern tanks, but the single trail and rigid carriage wheels made them awkward to transport. As a result, the Americans began to modernize these old guns, installing barrels from the M1897A4 on new M2A2 and M2A3 carriages with split trails and sprung wheels. There were also organizational changes put into effect. The Tank Destroyer Force was created as a fully fledged combat arm, on par with the Armor Force.

On a self propelled chassis

A meeting was held between representatives of the Ordnance Department and Army General Staff to determine the further development of anti-tank artillery. The main direction involved development of tank destroyer on a tank chassis, but this needed time. As an interim measure, the M1897A4 gun could be installed in the M3 halftrack APC. Its large troop compartment and lack of roof considerably simplified this conversion. The SPG had to be ready by the fall exercises.

Since the conversion was quite simple, the task of building the newly named 75 mm Gun Motor Carriage T12 fell to the workshops of the Aberdeen Proving Grounds. The troop compartment was emptied of benches, ammunition racks, and machine gun mounts. The two 30 gallon (114 L) fuel tanks in the front of the compartment were moved to the back. A welded steel box was installed behind the driver, which served as the pedestal for the gun mount and ready rack. The upper half of the M2A3 mount and M1897A4 gun with the existing gun shield were installed on top of the box. This whole assembly was designated 75 mm Gun Mount M3. The frame with the windscreen was removed, leaving only a folding armoured plate with vision slits.

75 mm Gun Mount M3

The gun had limited traverse angles: 19 degrees to the left and 21 to the right. The elevation was quite acceptable for an anti-tank gun: up to 29 degrees and down to -10 degrees. The vehicle carried 59 fixed rounds. 19 were carried in cylindrical slots under the gun and 40 more were stored in wooden crates on the floor of the fighting compartment. Auxiliary armament consisted of a 12.7 mm M2HB machine gun in the rear of the fighting compartment. The crew of the GMC T12 consisted of four men: the driver, his assistant (also the radio operator), commander/gunner, and loader.

Prototype of the GMC T12.

Conversion and trials of the GMC T12 only took a few weeks. As a result, some changes were introduced to the design. The most noticeable of them was a new gun shield. It now protected the crew not only from the front, but partially from the sides. The old M1897A4 gun sight was replaced with the new M33 telescopic sight. The crew was increased to five men by splitting the duties of the commander and gunner. Since this made the fighting compartment pretty tight, the machine gun mount was dropped.

GMC T12 with a widened gun shield. This is a different design than the ones used on production GMC M3s.

Production and development

After trials, the GMC T12 prototype was sent to the Autocar factory, which also received an order for 36 vehicles of this type. A contract for another 50 followed almost immediately. All 86 were finished in August-September of 1941. The vehicle was standardized as the 75 mm Gun Motor Carriage M3 in October. Mass production began at Autocar in February of 1942, and 2116 GMC M3s were built before April of 1943 (not counting the 86 GMC T12s). 

GMC T12 from B company of the 93rd Tank Destroyer Battalion during exercises held in November of 1941.

Stocks of M2A3 carriages ran out quickly, and so the older M2A2 mount was adapted for the conversion. The idea wa a sound one. The traverse range increased slightly (to 21 degrees left and right), but the depression was reduced to -7 degrees. Trials held at the Aberdeen Proving Grounds in the summer of 1942 were successful. The gun mount was standardized as the 75 mm Gun Mount M3 and the SPG as the 75 mm Gun Motor Carriage M3A1.

GMC T73 prototype.

Soon after the mount shortage was resolved, the Americans ran out of guns. The 75 mm M3 gun used on Sherman tanks was adapted to replace it. The gun was installed on an M3A1 for trials. The resulting SPG received the index T73, its gun was called the T15, and the mount was called T17. Trials showed that this conversion was workable, but production of the M10 tank destroyer was already underway. This SPG on a tank chassis armed with the long 76 mm gun was much more suitable as a tank destroyer than the M3 and M3A1. Production of the latter ceased. 1360 of these SPGs were converted back into APCs as soon as they left the factory, meaning that no more than 842 GMC M3/M3A1 were used as is.

Placement of the crew in the GMC M3.

Name of shell


Shell weight

Round weight

Muzzle velocity

Maximum range

Penetration at 1000 yards at 30 degrees (homogeneous/surface hardened armour)



6.79 kg

9 kg

609 m/s

12,680 m




6.77 kg

8.5 kg

609 m/s

9620 m




6.62 kg

8.27 kg

594 m/s

12,680 m


Blueprints of the GMC M3


The first batch of 36 GMC T12s was sent to the 93rd Tank Destroyer Battalion, one of the first tank destroyer units formed in the US Army. It took part in the large scale exercises held in South Carolina by the 1st Army in November of 1941 as a part of the Tank Attacker Group. In addition to the 93rd battalion, the group contained an infantry battalion, artillery regiment, company of Light Tanks M3, an engineering company, reconnaissance platoon, and even a squadron of Curtiss O-52 Owl observation aircraft. The concept that was being tested involved the tank destroyers attacking, rather than defending, actively seeking out and destroying enemy tank groups. Theoretically, its composition allowed this to be done. The conclusions were recorded in the appropriate field manual FM 18-5, which described how a tank destroyer battalion can be used in offensive actions against enemy armoured forces. It was assumed that the weak armour of the tank destroyers could be compensated by mobility and covering fire from friendly artillery. Combat in North Africa later showed that the GMC M3 was not at all suitable against an enemy armed with tanks.

A unit armed with GMC T12 tank destroyers.

The remaining 50 GMC T12 were delivered to the Philippines in November-December of 1941. They were assigned to the Provisional Field Artillery Brigade, which consisted of three battalions of four 4-gun batteries each. The SPGs took part in the defense of the Bataan peninsula. Some SPGs were captured by the Japanese and turned against their former masters in 1944-45.

The GMC M3 played a large role in training tank destroyer crews. This photo was made in April of 1942 in Indio, California.

Initially, tank destroyer battalions were supposed to be mixed, containing 37 mm GMC M6 on the Dodge truck chassis as well as the 75 mm GMC M3 and M5. The latter SPG on the chassis of the Cletrac tractor proved poor, even though it was standardized. As a result, tank destroyer battalions each received 36 GMC M3 and four M6.

GMC M3 in Tunisia.

Six tank destroyer battalions took part in Operation Torch, the landings in North Africa in November of 1942. Only one was armed with the GMC M10, the rest used the M3. The experience of these vehicles in Tunisia was not encouraging. M3 battalions tooh heavy losses at Sidi Bou Said and the Kasserine Pass. The most significant action that involved these tank destroyers was the Battle of El Guettar on March 23rd, 1943. The 601st Tank Destroyer Battalion numbering 31 GMC M3 and five completely useless GMC M6 helped the 1st Infantry Division to repel a counterattack by the German 10th Tank Division. It fought alongside the 899th Tank Destroyer Battalion that had the GMC M10. The result was the destruction of 37 German tanks, 30 of which were claimed by the 899th battalion. This success came at a great cost. Losses consisted of 21 GMC M3 (8 of which could be repaired) and one M6. The tank destroyers didn't hold back, firing 2740 shots and almost completely emptying their authorized capacity of 2844 rounds.

Elements of the 601st Tank Destroyer Battalion before the Battle of El Guettar. The vehicle closest to the camera is an M2A1 commander's halftrack.

Operation Husky (the landing in Sicily) saw the GMC M3 take a secondary role, chiefly in anti-tank companies of infantry regiments. A few of them were assigned to Ranger battalions as direct fire support.

GMC M3 in Sicily. The vehicle is equipped with a 12.7 mm machine gun.

170 GMC M3 were send to Great Britain as a part of Lend Lease. Starting with 1943, these vehicles were used in armoured car reconnaissance regiments as fire support. British M3s fought in the last battles in North Africa and continued fighting in Italy until the end of the war. Free French forces in North Africa received some of these vehicles from the Americans, but only used them for training. French tank destroyer units went into action with the GMC M10.

A pair of British GMC M3 from the King's Dragoon Guards, Italy, May 1944.

GMC M3s from the 27th Lancers in an indirect fire role. Italy, February 1945.

With the Marines

The history of the GMC M3/M3A1 with the USMC deserves a separate mention. USMC command planned on equipping two tank destroyer battalions in late 1941. To achieve this, they asked the army to loan them 60 tank destroyers in January of 1942, but received only 30. In the fall of 1942, when the army was in a better position when it came to anti-tank armament, the Marines received 219 GMC M3 and M3A1. One tank destroyer battalion was formed in September of 1942, but dissolved in December of 1943 without ever seeing combat. The USMC largely used these tank destroyers at the divisional level. One battery was assigned to the Special Weapons Battalion of Marine divisions. These batteries contained 12 SPGs organized into three platoons of four guns each. The Marines classified the GMC M3 and M3A1 as Self Propelled Mounts (SPM) and used them chiefly for infantry support.

A GMC M3 landing. The vehicle has two extra 12.7 mm machine guns in the fighting compartment and one extra 7.62 mm machine gun at the assistant driver's station.

GMC M3 landing at Cape Gloucester, December 1943. A 7.62 mm M1919A4 machine gun can be seen on the right side.

The Marines considered the M3 to be more suitable then the M10 for amphibious landings on account of their lower weight. The issue of enemy tanks in the Pacific was also far less pressing than in North Africa, Italy, or France. According to Steven Zaloga, the Marines' M3s first went into battle in late 1943 at Cape Gloucester (New Britain), although Richard Hunnicutt claims that they were first used in Guadalcanal back in August of 1942 and even provides photographs of their landings. In 1944 these vehicles were used on Bougainville Island and Saipan, and then at Iwo Jima and Okinawa in 1945. The GMC M3 and M3A1 was used by the USMC until the end of the war. Its career with the Marines turned out to be a lot longer than with the Army. They were mostly used as assault guns, suppressing Japanese positions. The SPGs were often modified with extra 12.7 and 7.62 mm machine guns, which came in handy in case of close quarters fighting. The most successful use of the GMC M3 in its original role was during the defense against an attack by the Japanese 9th Tank Regiment on Saipan on the night of July 17th, 1944. Japanese Ha-Go and Chi-Ha tanks were easy targets for the 75 mm gun. 

Marines with a GMC M3 on Bouganville.

USMC M3s at Iwo Jima.

Original article by Andrei Haruk.