That's an interesting idea. I wish I'd thought about all this a few years ago when I remodeled both kids' bedrooms. It would have been super simple then. Not so easy now...

A days worth of research has revealed that there is no Ammo that will be stopped by an interior wall that is considered lethal. Even Glaser Safety Slugs penetrate 3-4 walls (8 layers of drywall). Rubber bullets would be safe, and have the potential to be lethal, but that course seems as dangerous to the overall safety of the family as just using proper defensive Ammo.

I'm probably going to have to rely on having the bad guy stop the bullets. Shot placement and situational awareness are going to have to be good enough.