So I've got a 9mm two shot derringer I use as a backup carry piece. I've been told that, due to the short barrel length, there wouldn't be enough velocity to cause an HP to expand from such a handgun. My main concern, when considering defensive firearms, is safety. Not for myself, but, rather, for those around and in possible danger of an over-penetrating round. So I've started carrying lower calibers, but I do see the possibility of a need, in a unique occasion, in which some hopped up druggie might require a more persuasive nudge, thus the backup. My question is, if the velocity from my derringer is so much less, should I even bother worrying about over-penetration? Or how can I test this myself without paying a crazy price for balistic gel? Not sure what it costs, but guessing it's up there. Also, isn't the point of an HP to reduce chance of over-penetration? Is what causes the round to open the resistence received from its coursing through the air or the contact with its target? If it's the air, then wouldn't it be reasonable to assume that the impact with a body too could create that necessary resistence, allowing it to open and still serve its purpose, regardless of barrel length and/or velocity? If not then I understand. I'll fully admit it, I'm very inexperienced and unknowledgable when it comes to balistics and all this, hence my being here. How I'e come to my theory above is simply through a novices attempt at logical thinking. Hahaha Speak some knowledge at me; take me to school fellas!!! Thanks inadvance!