If you're worried about overpenetration but still want to ensure you hit the bad guys' vitals, I recommend #4 buck. I've posted my test of it here before but if you haven't seen it it's worth a look:
Birdshot may work okay but I wouldn't count on it. Kind of like how a 380acp might work in a pinch (both are a heck of a lot better than nothing)
but I have either #4 or #1 buck in my HD guns.