Lightweight, high-velocity rounds like the 5.56 tend to present less of an overpenetration risk than heavier, slower ones like the 9mm or 45ACP, even hollowpoints. Things like birdshot or Glasser frangible ammo can’t be counted on to reliably stop a threat so those aren’t a great solution either. Overpenetration is a serious concern with any firearm that is viable for self-defense, there’s no getting around it. Any time you pull the trigger you run the risk of killing an innocent person downrange of your intended target.