 Self-propelled tank destroyers, called "gun motor carriages" (as were any U.S. Army self-propelled armored vehicles mounting an artillery piece of heavy caliber) were similar to tanks, but were lightly armored with open-topped turrets.The tank destroyers were supposed to be faster and carry a more powerful anti-tank gun than tanks (although in reality tanks often received more powerful guns before tank destroyers did) and armor was sacrificed for speed.Armored Force and Tank Destroyer Force doctrine were developed separately, and it was not against Armored Force doctrine for friendly tanks to engage hostile tanks that appeared while attacking or defending.