Unless you are shooting very low poundage or are only getting say 40lbs of KE then there shouldnt be any problems with penetration on deer sized game. Like Dave said the new modern expandables penetration is right there with most fixed blade heads. I wouldnt shoot the old spitfire style heads if you paid me to, but the newer rear deployment heads are great. You can achieve more forgiving arrow flight with large cutting diameters andwithout loosing penetration. Will they replace a fixed blade head? Not in durability and strengthbut the new style mechanicals deffinetly have there place.