Much smaller light duty resetting sniper targets, just heavy enough for 223 for 50 yards & out. Something like MGM's mini pepper popper, but with a delayed reset system like the full grown Larue sniper target. Keep it light and under $200, I'd buy 5 today.
A adjustable gas block with repeatable settings the fits under and is operable from outside of the 13.2 rail.
Stealth AR10 upper (stripped and complete, both).