US Army pre-war infantry support doctrines emphasized the use of tank destroyers with open-top fully rotating turrets, featuring less armor than the standard M4 Sherman tanks, but with more powerful cannon. A 76 mm long-barrel tank cannon was fitted to the Sherman-based M10 and all-new design M18 designs, with the M18 being the fastest-moving American AFV of any type in World War II. Late in 1944, the Sherman-origin M36 appeared, equipped with a 90 mm cannon. With rotating turrets and good combat maneuverability, American TD designs generally worked well, although their light armor was no match for enemy tank cannon fire during one on one confrontations. Another disadvantage proved to be the open, unprotected turret, and casualties from artillery fire soon led to the introduction of folding armor turret covers. Near the war's end, a change in official doctrine caused both the self-propelled tank destroyer and the towed antitank gun to fall from favor in U.S. service, increasingly replaced by conventional tanks or infantry level antitank weapons. Despite this change, the M36 tank destroyer continued in service, and was used in combat as late as the Korean War.