Any one ever try 12ga rifled barrel shooting shot (8's or 9's)? My test show the rifling causes the shot to spread extreemly fast. 21' I get about 25" pattern. This stops alot of the slug effect.
And a belly full of 9's would be bad. It would be like trying to sew up a screen door. There are 600+ 9's in an 1 1/8 field load
And on dry wall any thing over a few feet it wont make it thru the third layer,
If you had no other option then to use .223/5.56 for HD I believe a varmint type bullet lighter then 55grs would be best as far as the least amount of over penetration. These bullets have very thin jackets and are designed to expend all there energy on some very small targets. I have shot water bottles in front of paper targets and only bullet fragments hit the paper. This was with 52gr SMK's out of a 24" barrel so I don't know if a shorter barrel would have the same results. The problem with this is I don't know how much "stopping power" a round like this would have. Although I keep a pistol close at night as the video with Michael Bane shows I believe the "best" inside the home defensive round is a 12 gauge with a 1 1/2 ounce load of #9's. This only takes into consideration "stopping power" and not how much harder it is to maneuver inside a home with a long gun.