One of the main American weapons produced as a counter to the low-flying aircraft was not of the same calibre as the other weapons in this study, for instead of using what are normally regarded as cannon calibres, the American solution used heavy machine guns with a calibre of 12.7 mm (0.5 in). This was the Maxson Mount, which used a combination of four 12.7-mm BrowningM2 heavy machine-guns on a single mounting with two guns on each side of a central pedestal-type housing. The proper service designation for this arrangement was Multiple Caliber .50 Machine-Gun Carriage M51.
The Maxson Mount was used on a variety of different carriages. One of the most common was a trailer towed by a light truck or even a Jeep. This trailer used twin axles, and in action legs could be lowered to the ground at each corner to provide increased stability when firing. The trailer also earned a number of batteries and a battery-charging set, for the Maxson Mount was electrically powered. The electrical supply was used for elevation and traverse, and the motors used were powerful enough to meet the most demanding calls made upon them by the gunner, who sat on the turret between the two pairs of machine-guns. The motors could move the guns from the horizontal to +60° in one second, and the turret could traverse at the same rate. In order to keep the two main batteries topped up at all times, they were normally kept on constant charge in action.
The combined fire of the four Browning machine-guns was sufficient to bring down any aircraft caught in their fire, despite the fact that the rounds carried no explosive payload. The guns were aimed using a naval reflector sight, but the tracer fired by the guns could also be used to assist aim and some gunners relied on the tracer alone to make fire control corrections.
The Maxson Mount was also used on halftracks as well as towed trailers. On both types of carriage the guns were supplied with 200 rounds each, fed into the guns from belts carried in enclosed chests mounted outboard of the guns. On some turrets the belts could be fed into the guns under electrical control but the normal gun action was more commonly used.
The Maxson Mounts were normally used to provide protection for convoys or mobile units against air attack, and after 1945 continued to serve with many armies. Many are still in use today, but recent years have seen a move away from the retention of the four machine-guns to a new configuration using two 20-mm cannon. Israel has adapted all the Maxson Mounts it. has in service to this new form, and Brazil is another nation taking the same path, Israel continues to use its modernized Maxson Mounts on halftracks, but there is a towed version as well.
Calibre: 12.7 mm (0.5 in)
Length:(guns) 1.654m (65.1 in)
Weight: in action 1087 kg (2,396 lb)
Elevation: -5° to+85°
Muzzle velocity: 884 m (2,900 ft) per second
Maximum effective ceiling: about 1000 m (3,280 ft)
Rate of fire: (cyclic, all guns) 2,300 rpm
The first of these new antiaircraft armored half-tracks was designated the multiple gun motor carriage M13. Placed in the rear compartment of the vehicle was an electrically powered, one-man armored turret armed with two air-cooled .50 caliber machine guns. It was designed and built by the W. L. Maxson Corporation and standardized in March 1943 as the twin .50 caliber machine gun mount M33. To allow the gun mount M33 to fire at minus 10-degree elevation the upper portions of the vehicle’s rear compartment were hinged to fold downwards if the need arose.
The White Motor Company built a total of 1,103 units of the 18,500-pound M13 between January and May 1943. Of that number, only 139 made it overseas with the U.S. Army. A large number of the remaining vehicles were converted into an improved version known as the multiple gun motor carriage M16. The main difference between the five-man M13 and the five-man M16 was the fact that there was a new gun mount, designated the M45, in the M16 that was armed with four air-cooled .50 caliber machine guns, doubling the firepower.
The M16 was approved for standardization in December 1942. Series production of the 19,000-pound vehicle by the White Motor Company began in May 1943 and concluded in March 1944 with 2,877 units being built. Counting additional halftrack personnel carrier M3s converted to the M16 configuration from other roles, there were 3,614 units of the vehicle assembled. The M13 was reclassified as substitute standard when replaced by the M16.
Prior to the invasion of France in the summer of 1944, the Chief Ordnance Officer of the First U.S. Army felt that there were an insufficient number of M16s in the inventory. What there was in the inventory in adequate numbers was a towed, four-wheel trailer fitted with the gun mount M45 armed with four air-cooled .50 caliber machine guns. He therefore ordered that these gun mounts be stripped from their trailers and mounted on surplus half-track personnel carrier M3s or the half-track car M2. First Army documents report that 332 such conversions took place. The converted vehicles were unofficially called the M16B and saw service with the U.S. Army until the war in Europe concluded, and then up through the Korean War.
The M16B differed from the M13 and M16 as the sides and rear of the vehicle’s rear compartment did not fold down to allow the gun mount M45 to engage targets at minus 10 degrees. To compensate for this, the gun mount M45 was raised 12 inches off the floor of the vehicle.
The twin gun mount M33 was also fitted to the International Harvester Company’s half-track personnel carrier M5, which resulted in the designation multiple gun motor carriage M14. There was a total production run of 1,605 units of the 9.6-ton M14 between December 1942 and December 1943, with all being allocated for Lend-Lease. When the quad gun mount M45 was fitted to the International Harvester built half-track personnel carrier M5, it was re-designated as the multiple gun motor carriage M17. The firm built 1,000 units of the 9.85-ton M17 between December 1943 and March 1944, all of which were supplied to the Red Army under Lend-Lease.
COMBINATION ANTIAIRCRAFT GUN HALF-TRACKS
Based on a request from the Coast Artillery Branch of the U.S. Army, the Ordnance Department came up with another antiaircraft vehicle, this time based on a modified half-track car M2. It was fitted with a manually operated gun mount that contained a single 37mm gun M1A2 and two water-cooled .50 caliber machine guns. The pilot versions of the vehicle were referred to as the multiple gun motor carriage T28. Based on testing of the pilot vehicles, the Coast Artillery Branch rejected the vehicle as being unstable and hence inaccurate. The Ordnance Department suggested to the Coast Artillery Branch that the weapon system be mounted on the larger chassis of the half-track personnel carrier M3, but this idea was rejected and the project was ended in April 1942.
In the summer of 1942, the U.S. Army identified a need for a dual-purpose weapon that could double as both an antiaircraft and antitank weapon. The Ordnance Department thought that a modified version of the multiple gun motor carriage T28, this time based on the chassis of the half-track personnel carrier M3, would be the perfect candidate for that role. The newest version of this weapon system was labeled the multiple gun motor carriage T28E1.
Eighty units of the T28E1 were built by the Autocar Company in time to take part in the American military invasion of North Africa in November 1942, codenamed Operation Torch. The U.S. Army Air Force did not enjoy air superiority in the North African Theater of Operations, and the T28E1 proved a most useful vehicle in warding off attacks by the German Luftwaffe. In the AAR Operations of the 1st Armored in Tunisia, Major-General E. N. Harmon stated: “The combination antiaircraft weapon of two .50 caliber machine guns and one 37mm gun was the most efficient antiaircraft weapon in the division.”
The Antiaircraft Artillery Board of the U.S. Army liked what they saw with the stopgap T28E1, and decided an improved version with two air-cooled .50 caliber machine guns in place of the two water-cooled .50 caliber machine guns would make more sense. That improved vehicle was referred to as the multiple gun motor carriage M15. Unlike the T28E1, the M15 had an open-topped armored shield around the front and sides of the weapons to protect the gun crew.
Following the fielding of the multiple gun motor carriage M15, a slightly different version appeared in service designated the M15A1. It had a different gun mount with the two air-cooled .50 caliber machine guns installed below the 37mm automatic cannon, as is seen in this photograph.
The first series production unit of the 10-ton M15 rolled off the assembly line of the Autocar Company in February 1943. Production of the seven-man M15 ended in April 1943 with a total of 600 units built. It remained in U.S. Army service until the end of the war in Europe when it was reclassified as limited standard.
There was also an improved version of the M15 constructed by the Autocar Company, referred to as the M15A1. The key external spotting feature of the 10-ton M15A1 was the mounting of the two air-cooled .50 caliber machine guns above the 37mm gun. On the M15, the two .50 caliber machine guns were mounted below the 37mm gun. The Autocar Company built 1,652 units of the M15A1 between October 1943 and February 1944. One hundred units of the M15A1 were supplied to the Red Army under Lend-Lease. The vehicle was reclassified as substitute standard when the war in Europe concluded.
Anti-aircraft variants of the M3 Personnel Carriers
T1E4/M13 MGMC – M3 based Multiple Gun Motor Carriage equipped with the Maxson M33 mount with 2 M2HB machine guns (5,000 rounds). The T1E4 prototypes had the hull sides removed for easy of working with the mount. These were reintroduced on production M13s. This was a development of previous T1s that had all been based on the M2 Half-track Car.
M14 MGMC – M13 MGMC variant, based on the M5 chassis. Supplied under lend-lease to Britain (5,000 rounds).
M16 MGMC – M3 based Multiple Gun Motor Carriage equipped with the Maxson M45 Quadmount (more specifically the M45D) with 4 M2HB machine guns (5,000 rounds).
M16A1 MGMC – Standard M3 Personnel Carriers converted to Multiple Gun Motor Carriages by removing rear seats and installing a Maxson M45 mount (more specifically the M45F, which featured folding “bat wing” gun shields on both sides of the mount over the machine guns). These vehicles are easily identified by the lack of the folding armored hull panels found on purpose-built M16s.
M16A2 MGMC – M16 MGMC variant, basically M16s brought up to M16A1 standard and with the addition of a rear door to the hull compartment. For existing M16s, this essentially meant a replacement of the M45D mount for the M45F mount.
M17 MGMC – M16 MGMC variant, based on the M5 chassis. Sent under lend-lease to USSR (5,000 rounds).
T58 – Similar to the M16/M17, the T58 featured the Maxon quad-mount fitted to a special electric powered turret. Prototype only.
T28E1 CGMC – M3 based Combination Gun Motor Carriage equipped with one M1A2 37mm autocannon (240 rounds) flanked by 2 M2WC machine guns (3,400 rounds). The original T28 had been based on the shorter M2 Half-Track Car chassis.
M15 CGMC – T28E1 variant, equipped with an armored superstructure on the turreted mount to provide crew protection, and switched to M2HB machine guns.
M15A1 CGMC – Reorganization of the weapons, with the M2HB machine guns being fitted under the M1A2 37 mm autocannon instead of above as on the M15.
T10E1 – Variant to test the feasibility of mounting US made copies of the Hispano-Suiza HS.404 20 mm cannon on modified Maxson mounts. All were later rebuilt as M16s. The original T10 was based on the shorter M2 Half-Track Car chassis.
40 mm Experiments – Various attempts were made to mate the 40 mm Bofors L/50 gun to the M3 chassis. In all cases the weapon’s recoil was too severe or the mounting too heavy, and the attempts were finally stopped with the adoption of the M19 MGMC on the M24 light tank chassis.
T54/E1 – Tested in 1942, the gun mount quickly proved to be unstable when fired, and the improved T54E1, which also added a circular armored shield and rear armor to the vehicle, could not fix the inherent problem. Prototype only.
T59 – A development of the T54/E1, fitted with outriggers to help stabilize the vehicle during sustained firing. Still proved to be too unstable for anti-aircraft use. Prototype only.
T59E1 – T59 fitted with the T17 fire control system. Prototype only.
T60/E1 – Similar to the T54 and the T59, but featured two .50 caliber M2 machine guns flanking the 40 mm cannon (the mounting’s designation was T65). The T60E1 featured an armor configuration similar to that of the T54E1. Suffered from the same stability issues of previous attempts. Prototype only.
T68 – Perhaps the most radical of the experiments, the T68 featured two 40 mm cannons, one mounted on top of the other, plus a stabilizer on top of the two guns. The recoil force proved to be too much for the mount, and the idea was abandoned. Prototype only.
M15 “Special” – Field conversions by US Army depots in Australia of standard M3s, not M15s, fitted with turreted 40 mm Bofors L/50 guns. These were the only successful mating of this weapon to the M3 chassis, and were used more for direct fire support than for anti-aircraft purposes.
The US Army did not use the Halftrack as a personnel carrier (APC) after WWII, and only the AA versions (Quad .50 M16A1 and Combo 37mm/twin .50 M15A1) appear on pictures from the Korean War.