"Over-penetration" only becomes an issue if something critical gets hit that wasn't supposed to after passing through something that was. Otherwise, if somebody wants to shoot a dumb projectile that whizzes right through a target doing minimal damage, it's up to them.
I don't need the government, a CCW instructor or Ayoob telling me that I can get in trouble if my bullets strike an innocent party. I was raised to not bring my troubles upon other people. If more people were, it would prevent many problems.
From what some folks on here are typing, one could easily assume that they'd carry the most highly penetrating bullets they could find in a densely populated area and have no qualms shooting somebody regardless of what was in back of the person. If you carry ammo that you know will shoot through several people and/or barriers and then you get into a shooting and an innocent person gets shot due to a round passing through the bad guy, IMO the shooter is negligent. The good guy did not initiate the proceedings and should not be the party with primary responsibility for that outcome, but if there were better options available and the shooter knowingly ignores them, then he or she bears at least some responsibility.
OTOH, let's say a good guy shoots a zombie and an innocent receives a pass-through round. The good guy shot an expanding round partially because he didn't want to risk his shot going through somebody and hitting somebody else, then IMO he has absolved himself of any responsibility for the outcome.
