Several pivotal design changes on the M4 Sherman tank were sparked by complaints about crew survivability, as well as general comfort for the crew.

Wet ammunition stowage reduced the risk of flash ammunition fires after a Sherman was hit from around 80 percent to 5-10 percent and in theory gave crews a couple more seconds to abandon the tank.

Early 75 mm turrets lacked an escape hatch for the loader. This crew member had to exit through the commander's hatch after he and the gunner had disembarked. If the commander's hatch was blocked for some reason, he had to worm his way out of the turret and make his way out of the tank through the driver or assistant driver's hatches. In the case of a flash ammunition fire where the crew had only seconds to escape, this could be fatal. In October 1943, a new turret entered production that incorporated a loader's hatch. The 76 mm turret incorporated a loader's hatch from the beginning, however.

Initially, the driver's and assistant driver's hatches of the Sherman were particularly small, and men had to turn almost sideways and move awkwardly in order to get in or out of the tank. This also made the evacuation of wounded or unconscious men more complicated. The small hatches also did not initially have a lock, and many crew members suffered injuries from falling hatches. A combination hatch lock and equilibrator spring were devised and installed at the various factories throughout the spring of 1943, with an Ordnance Department document stating that "no tank without this item to be accepted after 4/15/43".

Meanwhile, a more fundamental redesign of the Sherman was taking place that incorporated larger hatches. In February 1943, dimensions for the new hatches were submitted, but they could not be incorporated into the current 56-degree angled "small hatch" glacis of the Sherman.

Chrysler had developed a cast front upper hull section for the Sherman that sped up production, and this design was tested as a possible solution. Another design, submitted by Fisher, consisting of a 47-degree angled flat plate, was found to be ballistically superior, and was selected for production starting in November 1943. After November 1943, all Sherman types incorporated this new large-hatch hull design. The new large hatches incorporated internal springs and locking mechanisms.

Initially, the turret basket of the 75 mm turret was surrounded by a perforated steel mesh that made escape and retrieval of ammunition from the sponsons difficult as it only had a single exit hole. If the turret was turned just right and the commander's hatch blocked, the loader and gunner could be trapped in the turret. In order to retrieve ammunition from the sponson racks, the turret often needed to be turned to allow the loader access to the ammunition, wasting vital seconds in combat. One good example is the story of tank commander Lieutenant Raymond E. Fleig. Faced with a Panther tank rumbling toward him in the Hürtgen Forest, he ordered his gunner to fire the round already in the chamber, not realizing it was high explosive. The round did no damage, but the frightened Panther crew abandoned their tank. The enemy crew soon re-entered their tank, and fired a round at Fleig's tank, which missed. Fleig ordered his gunner to traverse the turret so his loader could reach the armor-piercing ammunition stored in the sponsons. The first round miraculously sliced off part of the Panther's gun tube; three more rounds destroyed the enemy tank.

The mesh was mostly removed from tanks that had it as part of the preparation for the D-Day invasion, and new "large hatch" 75 mm and 76 mm Shermans "skeletonized" the turret basket and did not have the mesh to begin with.

Sources:

Sherman Minutia Website

A Dark and Bloody Ground: The Hürtgen Forest and the Roer River Dams 1944-45, by Edward G. Miller