 When the newly-designed 76 mm gun, known as the T1, was first installed in the M4 in spring 1943, it was found to unbalance the turret, and the gun barrel also protruded too far forward, making it more difficult to transport and susceptible to hitting the ground when the tank traveled over undulating terrain.The barrel length was reduced by (from 57 calibers to 52), resulting in the M1 variant.Mounting this gun in the original M4 turret proved problematic, so the turret for the aborted T23 tank project was used instead for the definitive production version of the 76 mm M4 Shermans, along with a modified version of the gun known as the M1A1.