Bird shot, Buck shot, or slugs from a 12 ga. shotgun are all pretty much effective. Considering you only get a 1 inch spread per yard, bird shot at 5 yards is still going to knock the crap out of the BG assuming a center mass shot. The problem is that center mass shot. in an excited state people are amazed at just how crappy they shoot.
In any shotgun with any round it's inportant to put a light on the shotgun. It will help ensure your aim is more accurate. Most times the weapon will be needed at night. But even during daylight hours a bright light helps your aim.
Remember, in an OH CRAP situation, you are not you. YOU ARE SCARED!!
YOUR HEART IS PUMPING SO HARD IT'S ABOUT TO BURST!
In a gun fight, if your not hitting you're not winning.
Here is some food for thought in the youtube below.
http://www.youtube.com/watch?v=hrxkjRXk7m8