Except for most American designs, tank destroyers were all turretless and had fixed or casemate superstructures. When a tank destroyer was used against enemy tanks from a defensive position such as by ambush, the common lack of a rotating turret was not particularly critical, while the lower silhouette was highly desirable. The turretless design allowed accommodation of a more powerful gun, typically a dedicated anti-tank gun (in lieu of a regular tank's general-purpose main gun that fired both anti-tank and high explosive ammunition) that had a longer barrel than could be mounted in a turreted tank on the same chassis. The lack of a turret increased the vehicle's internal volume, allowing for increased ammunition stowage and crew comfort. Eliminating the turret let the vehicle carry thicker armor, and also let this armour be concentrated in the hull. Sometimes there was no armored roof (only a weather cover) to keep the overall weight down to the limit that the chassis could bear. The absence of a turret meant that tank destroyers could be manufactured significantly cheaper, faster, and more easily than the tanks on which they were based, and they found particular favor when production resources were lacking. After hard lessons early in the war, machine guns were mounted for use against infantry, but the limited traverse of the mounting meant that they were still less effective than those used on turreted tanks.