I can't believe that ANYONE HERE actually plans on shooting someone with a sub-standard load (birdshot) in order to "give the perp at least some chance to survive".
What complete and utter ignorance. First off, this is in complete violation of an etched-in-stone cardinal rule: DON'T POINT A GUN AT SOMETHING THAT YOU AREN'T INTENT ON DESTROYING.
Second... a living perp gets a criminal defense lawyer. I know that I certainly don't want any of that trouble.
Okay, here's my .02 cents (a quarter too late, I suppose):
If this is indeed for "home defense", measure the longest distance that you're planning on shooting. It may be somewhere between the longest hall in your home and from your house to your property line, I have no idea. Depending on your State, it will vary what you are planning on.
Anyways, get that distance out on the range and pattern your gun with all different sizes of Buck, from #4 up to #00 or #000. If you're looking at the longest hall in the house, you will most likely not notice much of a change in the pattern, as the shot will be moving in a column of supersonic metal. Either hit them with 8 or 9 .30-something caliber shot or a 27 count of .24 caliber shot.
You're responsible for EVERYTHING you put downrange, remember that when you're thinking about a home-defense load. If you over-pen with a slug and it kills your neighbor's baby, you're going up the river. If just one of your #000 shot does the same thing, you're going up the river. #4 Buck LE rounds were specifically developed to do the job of big buckshot at close ranges such as breaching homes or apartments calls for, while preventing over-penetration.
Look Here:
#4 Buckshot Description
Oh, & GOOD LUCK!