Similar to what smince said above, there are real action paintball markers out there and I can't imagine an airsoft bb hurting more than a paintball. Pain of being hit or at least being "marked" so you know you were hit gets my vote.
http://rap4.com/paintball/os/paintba...-c-21_158.html
http://www.packrat-toyz.com/RAM4/real_action_marker.htm