If you hit someone center-of-mass with a load of number 6 shot from a 12 ga., and were close enough that the pattern was under, say, 4 inches, I doubt that guy is going anywhere. But if the walls you're worried about are far enough away that the pattern would open up to say 8 inches or more before the shot hit it, I suspect it would't be deadly on the other side. Might still be a problem, but I'd take that over say a 9mm or a .223 going through the wall.