“My 9mm will kill the intruder, without going through the walls, penetrating the exterior walls of my neighbor and injuring/killing them. This is by design.”
Most handgun rounds will penetrate 3 or more sheetrock walls. Many will also penetrate cinder block and brick exterior walls. Shotgun (shot, 00, and slugs) will also penetrate multiple interior walls.
I’ve read several articles that the 5.56 round actually has the least penetration as the bullet will break apart by the 3rd sheet rock wall and fail to go through.
There are numerous videos on youtube showing penetration of the various calibers.
As BoT reiterates over and over again, "pistols are pistols, shotguns are shotguns and rifles are rifles". A rifle can do it all, including penetrating most soft body armor. No pistol or shotgun round can do this reliably. As for all the talk of overpenetration, unless you live in a stone castle any round can overpenetrate. A little preplanning does more to protect the innocent that all the nerf rounds in the world.