A part of the body inside the home, is considered inside during an unauthorized/forced entry. Where the perp did not belong...
If the perp was completely outside, then he might still be alive.
IANAL
Would you let the perp get inside comfortably then shoot him/her? Or would you shoot the perp in the act?
Well, first of all, I hope I am never in that situation. If I had a shotgun trained at the door and had already called 911, I think I would wait until the perp's body was through the door before I pulled the trigger. Maybe I am being stupid, but I think that's what I would do.