Safety slugs in the 9mm, barrier defeating rounds in the 5.56....
Not a misconception on my part, if either would do otherwise it would be a mis advertisement of the ammunition manufacturer.
I am sure there are rounds that will stop at a wall with the 5.56, and rounds that will penetrate them with the 9mm.
But they are not what is loaded in my weapons
Fair enough, and, IMO, a good choice in ammunition for the two.
Typically I would not consider safety slugs a good 9mm choice, but with the added velocity coming out of a carbine barrel that would be a great help to the round.
I apologize for jumping the gun and thinking that you were just assuming that the 5.56 would overpenetrate just because it was a rifle cartridge