Anything from OO buck to #4. Don't consider loading your own ammo or go for anything exotic such as the Dragon's Breath or Fleshettes. It might cause you more trouble in court and using what the police use is very good advice. Don't fall for the old "Racking the Slide" bit because when someone is sneaking around, you have to be as ready as you can. Having only a pistol grip on a shotgun is a terrible idea. You need all the control you can and pistol grips don't do it. Don't forget to call 911 first before you go hunting and be sure you tell them what you're doing. A better plan is just to hold up and let the cops do the work. It depends on the situation. I was checking out my garage after the door sprung open and set the alarm off.
I found out that garage floors are freezing cold when you're in your underwear and looking under the car for an intruder.
"It might cause you more trouble in court and using what the police use is very good advice. "
Hear, hear! Very good advice. If you actually kill someone with a handload of razor flechettes or a commercial round named "Terminator", the cops will probably let you go.
But, any time for the next seven or eight years, the wife, girlfreind, kids, or bookie of the dead burglar can sue you in civil court for wrongful death, loss of companionship, hardship, and lots of things only a lawyer can dream up.
You will be on the stand years after the event explaining to a scowling civil jury just why you loaded up your Mossberg 590 ASSAULT WEAPON with nine rounds named "PerpShredder II".
Standard, OTC hunting rounds are better from a legal defense standpoint.