As towed anti-tank cannon guns grew in size and weight, they became less mobile and more cumbersome to maneuver, and required ever larger gun crews, who often had to wrestle the gun into position while under heavy artillery and/or tank fire. As the war progressed, this disadvantage often resulted in the loss or destruction of both the antitank gun and its trained crew. This gave impetus to the development of the self-propelled, lightly armored "tank destroyer" (TD). The tank destroyer was usually based on the hull of existing tank designs, using either a gun integrated into the hull or a fully rotating turret much like that of a conventional tank. These self-propelled (SP) AT guns were first employed as infantry support weapons in place of towed antitank guns. Later, due to a shortage of tanks, TDs sometimes replaced the former in offensive armored operations.