5.56 does not usually over penetrate the round deflects and and goes sideways through the back wall.
Plus, subsonic rounds in a suppressed 300 blackout leave a significant advantage over a shotgun.
I wont push back too hard on the 5.56 not over penetrating,... but i was trained that drywall is concealment only.. not cover.
And if you need special mods and ammo to make sure it doesnt overpenetrate... then that complicates it.
For $500 bucks... he can have a reliable, easy point and blast home defense weapon.
Edit: 5.56 leaves the barrel at twice the velocity as a shell. I definetly would be concerned about overpenetration.
That would lead to hesitation.. which is not good in that situation.