Every house layout really has just one or two "most likely" lanes of fire, given your floorplan, points of entry, and living arrangements. Create some kind of backstop behind those "most likely" areas, and make sure everyone in the home has something solid to duck behind in their own bedroom(s), and you're a lot safer than you were before.
That's basically what I'm trying to do on the relative cheap. Given my house and bedroom layout, I think a "bullet stop" on the back/side of one or two of my closets should be enough to protect my next door neighbor in the event of my needing to defend myself. I'm just looking for the lowest cost option (as opposed to the expensive options that I'd get around to eventually....).
If I can't stop
a 9mm SD round, what's the cheapest/thinnest material that will take enough out of it so the neighbors don't have to worry?