76 mm gun M1

Design and development

Development of a better weapon than the 75 mm gun was foreseen before the U.S. had combat experience with well-armored German tanks. The original Military specifications of 11 September 1941 for the M4 tank allowed for the mounting of numerous weapons including the 3 inch gun. The first specimens of the weapon that was to become the 76 mm Gun M1 were being evaluated in August 1942 while the U.S. did not enter the ground war in the European/Africa region until Operation Torch in November 1942.

The 3 inch gun was considered too heavy at about 1,990 lb (900 kg). New stronger steels were used to create a weapon weighing about 1,200 lb (540 kg). It was a new gun with a breech similar to that of the 75 mm M3 Gun but with a new tube (barrel and cartridge chamber) design to accommodate a new cartridge. It fired the same projectiles as the 3-inch (76 mm) M7 gun mounted on the 3in Gun Motor Carriage M10 tank destroyer and towed 3-inch Gun M5 anti-tank gun, but from a different cartridge case. The "76-mm" designation was chosen to help keep the supply of ammunition from being confused between the two cannon. The 76 mm also differed in that successive models received a muzzle brake and faster rifle twist.

Aberdeen Proving Grounds began evaluations of the first test guns designated the T1 around August 1, 1942. The first test guns had a bore length of 57 calibers and when tested on an M4 Sherman tank, it was then found that the long barrel caused balance problems. Another T1 test gun was produced with the barrel shortened and a counterweight added to the breech to improve balance. The reduction in length by 15 inches (38 centimetres) did not reduce performance; penetration was the same as the 3 inch gun.

By August 17 the Ordnance Department had classified the test gun with the shorter barrel as the 76-mm M1 and set up the precedent for the designation of M4 tanks armed with the gun to include “(76M1)”.

Tests of production M1 guns revealed that the cannon with its counterweight had issues with binding when trying to rotate the turret when the tank was resting at a steep angle. An 800 lb (360 kg) storage box was added to the turret rear to improve balance, with evaluations held in early 1943 and the final report tendered April 1943. This worked but was rejected by the Armored Forces due to the turret being cramped.

A more satisfactory mounting was found in August 1943 by utilizing the turret design of the T23 tank on the M4 chassis to carry the 76 mm gun. The 76 mm M1A1 version of the cannon was created, having a longer recoil surface to also help with balance by permitting the placement of the trunions further up front.

Acceptance by the tank destroyers

The new M18 Gun Motor Carriage armed with the 76 mm was the result of the Tank Destroyer's request for such a vehicle and as such they accepted them.

Acceptance for production by the tank forces

By August 1943 the Ordnance Department M4 tank armed with 76 mm in the modified T23 turret was finally ready for production. A proposal was made by the tank forces for a test run of 1,000 tanks for combat trials and if that was successful then all of the M4 tank manufacturing capacity would be devoted to the 76 mm gun, but this was changed to a rate that would equip 1/3 of the M4 tanks with the 76 mm gun.

The production proposal was part of a memo in September 1943 pointing out various flaws of the gun that made it less desirable for tank use: muzzle blast, the high explosive shell, ammunition handling, and ammunition storage. Summed up, the 76 mm offered about 1 inch (25 millimetres) of added armor penetrating power for a possible loss of some ground attack fire-power. In a meeting in April 1944 held to discuss the assignment of the first production M4(76M1) tanks received in Britain to units, a presentation comparing the 76 mm to the 75 mm went over similar points, adding that the 76 mm was more accurate and did not have an appropriate smoke round.

Muzzle blast

The 76 mm obscured the target with smoke and dust. This could prevent the gunner from seeing where the projectile struck.

The Ordnance Department reduced the amount of smoke by using a long primer that gave a more complete burn of the propellant before it exited the barrel. The revised ammunition began to be issued for use in August 1944.

Later production guns were threaded for a muzzle brake to redirect the blast left and right (M1A1C and M1A2). This was tested in January 1944, authorized in February 1944 with production starting June 1944. The threads of those without a brake were covered by a protector (visible in many pictures).

For those vehicles that did not have a muzzle brake, once the Armored Force began to accept M4s, it was recommended that tank commanders stand outside the tank and “spot” the strike of rounds to guide the gunner.

High-explosive capacity

The situation with the high-explosive shell was that the 3 inch M42 projectile for the 76 mm gun carried a filler of about 0.9 lb (0.41 kg) of explosives while the 75 mm gun M48 high explosive projectile carried 1.5 lb (0.68 kg). Far more high explosive ammunition was used by tankers than armor penetrating types, the ratio being about 70% HE, 20% AP and 10% smoke overall, The ratio could vary by unit: From August 3 to December 31, 1944 the 13th Tank Battalion fired 55 rounds of M62 APC-T armor piercing versus 19,634 rounds of M42 high explosive.

Smoke ammunition

The M88 smoke round for the 76 mm provided a “curtain” of smoke. The tankers found the 75 mm M64 WP (White Phosphorus) smoke projectile useful not only for providing smoke coverage but also attacking targets including enemy tanks. Some units equipped with the 76 mm preferred to maintain a 75 mm armed tank on hand to provide the M64 WP projectile.

Round size

It was thought that the longer and heavier 76 mm might hamper handling inside the tank's turret, slowing the rate of fire. This may have been more of a concern than was warranted: on April 22, 1945 an M4 76-mm tank came suddenly onto a strange vehicle and the “...76 roared twice in rapid succession...” adding a friendly British scout car (waiting to ambush passers-by) to the tank's kill tally before (as the British gunner stated) “..I could lay my hand on the trigger.”

It was also thought that the longer 76 mm would reduce ammunition capacity. The 76 mm was first tested on the M4A1 series tank which carried 90 rounds of 75 mm ammunition, while most other models carried 97 rounds of 75 mm. The 76 mm cartridge reduced this to 83 rounds. By late 1943 the Army had adopted the wet storage system to reduce fires and for the 76 mm gun this provided 71 rounds of ammunition, while the 75 mm could carry 104 rounds. Storage depended on organization: The 76-mm T72 Gun Motor Carriage, designed to mount the 76-mm on the M10 GMC chassis in a T23 turret lightened for the job, carried 99 rounds (but not in wet storage).

Variants

A muzzle brake was tested in January 1944, authorized in February 1944 with production starting June 1944. Not all guns received them. The threads of those without a brake were covered by a protector visible in many pictures.

Ammunition

While the 76 mm had less High Explosive (HE) and smoke performance than the 75 mm, the higher-velocity 76 mm gave better anti-tank performance, with firepower similar to many of the armored fighting vehicles it encountered, particularly the Panzer IV tank and StuG assault gun vehicles. Using the M62 APC round, the 76 mm gun penetrated 109 mm (4.3 in) of armor at 0° obliquity and 1,000 m (3,300 ft), with a muzzle velocity of 792 m/s (2,600 ft/s). The HVAP round was able to penetrate 178 mm (7.0 in) at 1,000 m (3,300 ft), with a muzzle velocity of 1,036 m/s (3,400 ft/s).

The M42A1 High Explosive shell contained a 0.86 lb (0.39 kg) explosive filler of TNT or a 0.85 lb (0.39 kg) mixture of 0.08 lb (0.036 kg) of cast TNT and 0.77 lb (0.35 kg) 50/50 Amatol. A reduced charge load existed with a velocity of 1,550 ft/s (470 m/s) and range of 8,805 yd (8,051 m).

The standard M62A1 Armor Piercing Capped projectile was of the APCBC design.

The substitute standard M79 Armor Piercing solid monobloc shot had no filler, windscreen, or penetrating cap.

The M88 H.C. B.I. Smoke Shell contained a filler of H.C. Based on a British design, it was intended to provide a slow-release “curtain” of smoke versus the exploding white phosphorus shell available to the 75-mm and other cannon originally designed for artillery spotting but which could also cause damaging burns.

The M26 brass cartridge case was used for all loaded rounds, with a weight of 5.28 lb (2.39 kg) and length of 21.3 in (54 cm). It was an entirely different case from the 3-inch MKIIM2 case used for the 3-inch M3 anti-aircraft gun and 3-inch M5, M6, and M7 guns used on the a towed anti-tank gun, M6 heavy tank, and M10 Gun Motor Carriage. The 76-mm chamber capacity varied by projectile (also given is the capacities for similar 3-inch rounds to illustrate the size differences):

The 3 inch cartridge was not completely filled by the propellants used; a distance wad was used to keep the propellant pressed against the primer end. By way of comparison the 75 mm M3 gun had a chamber capacity of about 88 cu in (1,440 cc) for the M61 armor piercing projectile and about 80 cu in (1,300 cc) for the M48 high explosive projectile and the British 17pdr 300 cu in (4,900 cc).

Alternatives to the 76 mm M1

The 76 mm M1 was a project initiated by the Ordnance Department itself. Various entities suggested other weapon options which were not pursued.

US service

The 76-mm gun saw first use in a test batch of M18 Hellcat gun motor carriages in Italy in May 1944, under their development number T70. Only the US used the M18. The moderate performance of the 76-mm cannon by 1944 standards was one of three reasons the plans for M18 production were cut from 8,986 to 2,507, of which 650 were converted to unarmed utility vehicles. An experiment was performed mounting the 90-mm armed M36 turret on an M18 to provide more firepower than the 76-mm.

The first M4 tanks armed with 76-mms intended for combat were produced in January 1944. Tanks equipped with the cannon began arriving in Britain in April 1944. The issue with muzzle blast had not been addressed and higher level commanders had doubts about the use of, let alone need for, the new weapon. The medium-velocity 75 mm M3 gun, which first armed the standard M4 Sherman, was quite capable of dealing most of the German armored fighting vehicles met in 1942 and 1943 and had better ground fire capability and fewer issues with muzzle blast. It was not until July 1944 that a call for M4s armed with 76s was put out in France after unexpectedly high losses by US tank units and the arrival of numerous Panther Tanks on the US sector of the front.

Deliveries of the 76-mm armed tanks lagged such that by January 1945 they made up only 25% of the tanks in Europe. Plans were made by field units to directly replace the 75s on some tanks using a weight welded to the turret rear to balance it. A prototype was built but the supply of ready-made tanks increased and that project ended.

The 75-mm armed M4 tanks were never completely replaced during the war with some units in Europe still having about a 50/50 mix. Units in Italy readily accepted the 76-mm but were never shipped as many as desired. The US units in the Pacific Theater relied mainly on the 75-mm gun. The 76-mm armed M18 did see use in the Pacific late in the war.

British service

The UK had developed a more effective anti-tank cannon before 76 mm gun became widely available. Although only slightly longer at 55 calibers, their Ordnance QF 17 pounder (76.2 mm) anti-tank gun had a much larger 76.2×583mmR cartridge case which used about 5.5 lb (2.5 kg) more propellant. The anti-tank performance of the 76mm was inferior to the British 17 pounder, more so if the latter was using APDS discarding sabot rounds, though with that ammunition the 17-pdr was less accurate than the 76mm.

The 76 mm gunned Shermans supplied to the British were only used in Italy or by the Polish 1st Armoured Division in North-West Europe. The British and Commonwealth units in north-west Europe supported their 75 mm gunned Shermans with QF 17 pdr armed Sherman Fireflies.

Russian service

The first 76-mm-armed Shermans started to arrive to Red Army units in late summer of 1944. In 1945, some units were standardized to depend mostly on them, transferring their T-34s to other units. Parts of the Polish First Army also briefly used M4A2 (76 mm) tanks, borrowed from the Red Army after heavy losses in the conquest of Danzig.

Korea

By the end of 1950, more than 500 76mm gun M4A3E8 tanks were in Korea. These 76-mm armed Shermans served well in the Korean War and, having better crew training and gun optics, had little problem piercing the armor of North Korean manned T34/85 tanks when firing HVAP rounds, which was amply supplied to units. Some 76-mm armed M4s and M18s were distributed around the world and used by other countries post-war. The 76-mm gun was sometimes replaced by a more powerful weapon.

Middle East

Some 76mm gun Sherman tanks were used by Israel and Arab countries.

Balkans

Some M4A3E4, retrofitted with the M1A1 76 mm gun, were used during the 1990s conflict in former Yugoslavia.

Indo-Pak wars

Pakistan bought 547 M4A1E4(76)s during the 1950s and used them in 1965 and 1971 wars.

Uganda

Uganda purchased a few ex-Israeli M4A1(76)W and used them during the Idi Amin regime.

Vehicles mounted on

With British Commonwealth designations in parentheses:

Towed variant

From 1943, at the instigation of the head of the Armored Force General Jacob Devers, US Ordnance worked on a towed anti-tank gun based on the barrel of the M1, known as "76 mm gun T2 on carriage T3". Later interest in the project declined and the program was officially cancelled in 1945.