The general consensus is to pattern your gun with your chosen load. Personally I prefer buckshot for HD but some people use larger bird-shot. I don't agree with that but its not up to me. I have a lot of #4 and 00 for my 590A1 standard riot style gun. Some people feel #4B is to small, but I'm comfortable with it, it will penetrate around 11"-15", which is fine for me. When I am in the truck or the canoe, or tramping around in the woods, I usually carry it loaded with slugs.
But pattern and practice is what makes it.
Edit: Read the Defensive shotgun ammo sticky at the top of this page. Almost nobody recommends birdsh*t for HD/SD. It can create ghastly yet shallow wounds, failure to penetrate heavy clothing, and does not carry enough energy per pellet to be effective. I've had #6 shot fail to penetrate thru-and-thru on squirrels and rabbits, at average shotgun range which for me is 10-30 yards. If it fails at that task, it can most certainly fail at the worst moment.