The M15 Combination Gun Motor Carriage was an effective anti-aircraft weapon that carried a 37mm cannon and two .50in machine guns on the back of an M3 half-track and that saw combat with the US Army from the start of Operation Torch in November 1942 until the Korean War.
Work on mounting anti-aircraft machine guns on a half-track began in October 1940 with the T1 Multiple Gun Motor Carriage, the first in a series of designs armed with twin .50in machine guns. Work on the T1E2, the precursor of the production M13 MGMC began in November 1941 and the M13 was accepted for production in July 1942.
In September 1941 work began on a project to mount a heavier anti-aircraft gun on a half track, with the designation T28 Combination Gun Motor Carriage. This carried an M1A2 37mm autocannon and two .50in machine guns (mounted above and to the sides of the cannon) on a large open mount carried on a rotating platform on the back of an M2 half-track. The mount was too large to fit within the normal armoured sides of the half-track and so the sides and rear walls of the fighting compartment were removed, leaving an open platform.
The twin .50in machine guns were usually used with tracer rounds to allow the gun crews to hit their target. Once the machine gun fire was accurate the 37mm gun would open fire, hopefully making more effective use of the more limited supply of 37mm ammo.
The Coast Artillery Board, which was then in charge of the development of anti-aircraft weapons, liked the overall design and gun mount, but preferred machine guns to cannon. In the spring of 1942 the T28 was cancelled and work moved onto the T37 Multiple Gun Carriage, which carried four .50in machine guns on the same mount.
In June 1942 the Armored Force asked for a crash programme to develop a mobile anti-aircraft weapon for use in the upcoming North African campaign. The T28 project was revived as the design had reached a stage where it could easily be rushed into production. The only major change to the design was a switch from the M2 half-track to the slightly longer M3 half-track, and the vehicle was then accepted for production as the expedient standard T28E1. Eighty T28E1s were completed by August 1942, and most of them went to North Africa.
The Tunisian campaign was one of the few occasions when the US Army fought the Germans without air superiority and so the anti-aircraft weapons came into their own. The T28E1 claimed 78 victories in three months during 1943, with 39 German aircraft shot down during the fighting in the Kasserine Pass alone. At least one T28E1 was captured by the Germans during this battle and later used as a transport vehicle. The T28 was also used during Operation Husky - the invasion of Sicily and in the invasion of Italy. Some were used during Operation Dragoon, the invasion of the South of France in August 1944.
In February 1943 production of the T28E1 resumed. It was reclassified as substitute standard and redesignated as the M15 Combination Gun Motor Carriage. The main difference between the T28E1 and the M15 was the introduction of a five-sided gun shield - this had an open back, flat sides, diagonal front corners and a flat front with a gap for the gun and provided some protection for the gun crews. The M15 also saw air cooled machine guns introduced, replacing the water-cooled models of the T28E1. A total of 680 M15s were produced between February and April 1943.
The most numerous version of the vehicle was the M15A1. This had a number of improvements, including a new M3A1 gun mount with the twin machine guns below the 37mm gun instead of above it, a modified shield and new gun sights and a strengthened chassis.
1,652 M15A1s were built between October 1943 and February 1944, 1,052 in 1943 and 600 in 1944. The vast majority of these vehicles went to the US Army, although 100 were exported to the Soviet Union under Lend-Lease.
Service Record (M15 and M15A1)
The M15 was used alongside the M16 Multiple Gun Motor Carriage. Armoured divisions received anti-aircraft artillery weapons companies, each of which had eight M15s and eight M16s. They were also used in anti-aircraft artillery weapons battalions at corps and army level, each of which had thirty-two M15s and thirty-two M16s. These were used to protect high value targets such as bridges, head-quarters or rail junctions.
The M15 began to enter combat in Italy in the autumn of 1943, at first operating alongside the earlier T28E1 and the twin machine gun armed M13 Multiple Gun Motor Carriage. The M13s were later replaced by quad .50in armed M16s, but the M15 remained in use throughout the war.
The M15 was used during Operation Dragoon, the invasion of the south of France of August 1944, where it was initially used in the anti-aircraft role as the Luftwaffe attacked the beachheads.
The M15 took part in the D-Day landings, with some landing on D-Day itself. The M15 was then used for the rest of the campaign in north-western Europe. There was rarely a major threat from the Luftwaffe, and so the M15 was often used as an infantry support weapons. Some were involved in the battle for Bastogne during the Battle of the Bulge. This period also saw the last major clashes with the Luftwaffe, and so for a brief spell at the end of 1944 and early in 1945 the M15 reverted to its original anti-aircraft role.
The M15A1 saw limited service in the Pacific. Some were used during the American return to the Philippines, especially on Luzon, and during the battle of Okinawa in the summer of 1945.
The Ordnance Department made a number of attempts to mount a 40mm Bofors gun on a half-track without success - the heavy gun produced too much recoil to be an effective anti-aircraft gun on the light half-track chassis. However there was one successful conversion, with the unofficial name of 'M15 Special'. This was produced at the Coopers Plains 99th Ordnance Depot near Brisbane, Australia. The Bofors gun was installed in an eight-sided turret on the back of a variety of half-tracks (probably not including the M15, despite the unofficial name). This conversion was no more stable than the official ones, but it was never intended to use it as an anti-aircraft weapon. The modified weapons were issued to the 209th AAA Battalion, which used them as an infantry support weapon during the fighting on Luzon.
The M15 was one of the few half-track based weapons to remain in use after the Second World War. It saw combat during the Korean War, alongside the M16, where once again it was used as an infantry support weapon. The M15 had largely been phased out by the end of the Korean War.