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Long ago designers of armored vehicles realized that if you sloped armor it would block heavier shells than the same amount and type of armor arranged vertically. The Russian T-34 tank, and German Panzerkampfwagen VI Panther all have heavily sloped front glacis plates which make the tanks much tougher. But few tanks are pure slopes. Tanks require a driver who needs to see, which he does best with his head above his hatch. The turret can't take his head off either when it moves. So sometimes the compromises inherent in designing an armored vehicle lead to what is known as a shell trap.

A shell trap occurs when the shape of the vehicle tends to trap any round striking its armor, and guides it to a weaker spot in the armor. For example on the Sherman tanks the original hull is inclined at 63 degrees, a lot of slope which makes the sherman's 1 1/2 to 2" of armor more effective. But the tank's driver and assistant driver were each given a direct vision slot in the tanks original design, which led to a blister on the hull to accomodate the heads of each crewman. If a Panzer IV scored a hit on the sherman's front hull in the wrong spot, the shot would be guided up to the driver's direct vision slot, where the armor is weaker. A hit that might not have penetrated now may get through, and decapitate the driver. The bulge created by the direct vision slot is a shell trap, a place where the shape of the armor trapped a shell to bad end.

This weakness in the Sherman's armor was discovered in 1942 when early model Shermans went up against the Afrika Korps. In 1943 an inch of extra armor was added to cover the direct vision slot (and the ammo boxes in the sponsons). Later models of the M4A3 the hull shape was changed, and the glacis slope reduced to 47 degrees to eliminate this hull trap, even though that reduced the overall slope of the armor. This shows how much a shell trap can affect the hull's integrity.

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