AR M-4 style build question

[MachIVshooter]

The grenade launcher might be a bit of a issue with the BATF. Maybe if I use it to lob tennisballs.

I don't have any barrels with a launcher cut (the two that had it I reprofiled into pencil and sub-pencil), but if you wanted to, this is a 100% legal non-firearm (unregulated) option:



Do be aware, though, that the use of chamber inserts to make it use shotshells, or loading 37mm rounds with kinetic or explosive projectiles turns this flare launcher into an NFA weapon.