The HK91 is banned in NJ by name. You can however purchase a PTR91 or a different type of similar clone. Also, flash hiders and threaded barrels are illegal in NJ on a semi-auto rifle with a detachable magazine. You must perminately pin/weld a muzzle brake on the barrel so it cannot be removed. Unless you can find a way to mount a BFA onto your muzzle brake, I’d say it’s not in the cards.