I talked to a Viet Nam vet at the range a while back. He told me they used #4 or #6 shot for breeching and it was quite effective for anything inside 50 feet as well. Said you really needed buckshot after 50 feet or so but nothing kept it from working closer.
I keep my house shotgun loaded with 3" 15 pellet 00 buck now, but I wouldn't be afraid to trust #4 buck for inside the room distances. I've trusted it before but thankfully never had to try it.
Proud member of the NRA and Texas State Rifle Association. Registered and active voter.