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The Medium Tank M4A1 (76M1) was the first attempt to fit a more powerful gun in the Sherman tank, but was abandoned after objections by the Armored Force.
The Ordnance Committee was aware of the need to improve the firepower of the Sherman as early as September 1941, when the 3in gun M7 was suggested as an alternative gun. This 76mm gun had a much longer barrel than the 75mm gun of the M4, and thus fired a higher velocity armour piercing shell. It had been accepted as the gun for the Heavy Tank M6, but it was believed to be too heavy for use in a medium tank.
Work thus began on the 76mm gun T1, a lighter version of the 3in gun. This used a lighter barrel than the 3in gun, but as first designed kept the same 2,600ft/ sec muzzle velocity. The original version of the gun had a length of 57 calibres. The 76mm gun T1 used the same breech ring as the 75mm gun, and could thus be installed in the standard gun mount used on the Sherman.
Two 76mm guns were built during the first half of 1942, and went to the Aberdeen Proving Ground in August. One of these guns was installed in a standard M4A1. Even with the weight reductions from the 3in gun, the T1 was unbalanced in the Sherman. In order to compensate for this 15in was cut off the muzzle end of the gun, reducing the length to around 52 calibres, and extra weight was added at the breech end. In this configuration the gun was more satisfactory, and the Aberdeen Proving Ground reported that it could indeed be used in the M4.
On 17 August 1942 the Ordnance Committee recommended that the M4 with 76mm gun should be accepted as Substitute Standard, while the T1 gun became the 76mm gun M1. Plans were put in place to produce 1,000 tanks armed with the 76mm gun
Tests continued at Aberdeen. Extra weights were added to the recoil guard to fully balance the gun and allow the gyro-stabilizer to work properly. An extra 800lb counterweight also had to be added at the back to the turret to compensate for the heavier gun and allow the powered traverse systems to work properly when the tanks wasn't on level ground.
Twelve M4A1 (76M1) tanks were built by the Pressed Steel Car Company, and used for further evaluation by the Armored Force Board, the Tank Destroyer Board and at Aberdeen. These tanks began to arrive in February 1943, and included most of the modifications suggested by early tests. These tests were completed by 5 April 1943.
At this point in the war the US Army suffered from a flawed vision of armoured warfare, in which US tanks were not meant to take on German tanks. Instead that was the role of the Tank Destroyers. The Armored Force was thus not that interested in tanks with improved anti-armour capabilities. It objected to the M4A1(76M1) on the grounds that it was a 'quick fix' design, and there wasn't enough space in the turret after the larger gun and its various counterweights was installed. On 3 May 1943 the army cancelled the M4A1 (76M1). Three of the existing tanks were to be kept as test vehicles, one at each of the test locations, and the other nine were turned into standard 75mm Shermans. This didn’t end work on a 76mm armed Sherman, and attention now moved to on to the Medium Tank M4E6.