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The Medium Tank M4E6 was the second attempt to install a 76mm gun on a Sherman tank, and saw the introduction of a number of features that made their way into production tanks.
The 76mm gun M1 had been developed in 1942, and was a lighter version of the 3in gun M7 that was intended for use in heavy tanks. It was successfully installed in a standard M4A1 turret, although only after a number of counterweights had been added, and in August 1942 was recommended for acceptable as Limited Standard, with plans to built 1,000 76mm armed tanks alongside the standard 75mm Sherman. Twelve test vehicles were produced, but the Armored Force objected to the cramped turret of the Medium Tank M4A1 (76M1), and on 3 May 1943 the project was cancelled.
The same meeting that cancelled the M4A1 (76M1) also ordered work to begin on an improved M4, using the turret designed for the Medium Tank T20 series, itself designed as a possible replacement for the M4 Sherman.
The M4E6 used the combination hull, with the welded middle and rear and cast front sections, as used on late production M4s. The cast turret came from the Medium Tank T23. It had the same circular split hatch as the M4 for the commander and a rectangular split hatch for the loader. A new combination gun mount T80 was used, which had a 3.5in thick gun shield, and no rotor. It used the 76mm gun M1A1, which was better balanced than the M1 gun.
One of the biggest problems with the standard M4 was the vulnerable ammo storage, with the 75mm shells stored in the sponsons, where they were very vulnerable to enemy fire. On the M4E6 shell storage was moved below the sponson line, so extra protection was provided by the suspension. The turret basket was removed, to make it easier to reach the new shell storage. Finally 'wet' storage was introduced, with water tanks surrounding the shells to reduce the risk of fire. Seventy one 76mm shells could be carried.
Two M4E6s were built at the Detroit Tank Arsenal and one was sent to the Aberdeen Proving Ground, arriving in July 1943. After tests there it was moved to Fort Knox for firing tests.
The M4E6 now ran foul of the complex US Army structure. On 17 August 1943 the Armored Board recommended immediate production of the M4E6. Army Ground Forces asked for 1,000 M4E6s and the Chief of Ordnance was ordered to end production of the standard 75mm armed M4. At this point the Armored Force objected to the entire programme. In their view tanks weren't meant to fight other tanks, so the extra armour piercing capabilities of the 76mm gun were of little value. The tank was seen as an exploitation weapon, and the HE shell was thus the most important. Here the 75mm gun did have the edge, as its HE shell carried a significantly greater quantity of explosives than the 76mm equivalent.
Despite the Armored Force's objections, in the summer of 1943 work began on preparing the 76mm armed Sherman for production. Production of the 75mm armed M4A34 would continue for a little longer, but the M4, M4A1, M4A2 and M4A3 would be replaced by the M4(76)W, M4A1(76)W, M4A2(76)W and M4A3(76)W respectively. The new range of M4s was completed with the M4 (105mm Howitzer) and M4A3 (105mm Howitzer). All would use wet shell storage.
The M4E6 at Fort Knox continued to be used in tests. A muzzle brake and changes to the ammo dealt with a problem caused by the extra muzzle blast of the 76mm gun, which quickly obscured targets. Most later 76mm guns were equipped with a muzzle.