I have seen the NAP expandables open on grass, twigs, thick paper, about anything they hit. I don't think the concern should be will they open, biggest concern is if they hit something and open before they get to the target. Don't really think that would be a concern because a fixed blade would have the same result hitting an object before the target, bye bye arrow! with the kenetic energy and speeds of bows today, I think it would be adviseable to use expandables. A lot of guys are getting pass throughs with carbons and high speed bows, and that means one cut and one cut only. The expandables by far have a larger cutting diameter, thus more damage on a pass through. However if I were shoot aluminums or lower poundage slower bows I would stick with a fixed because your most likely leave an arrow to turn inside and probably not going to pass through. You hear all kinds of horror stories, but I've seen both bad and good for both. Most of the situations that each fail, 90% chance it was the shooter taking a poor percentage shot, not the broadhead. Shoot them and then make a decision on your personal preference...What's best for you and what you feel most confident with!