AR-15 upper assembly 5.7 x 28 fn 16 new also called panzer ar57 ult upper assembly 5.7x28mm 16 AR57 upper assembly 5.7 x 28 fn 16 from us is a factory new upper and has a 16" lightweight barrel with M-LOK rails.The barrel is 16 inches with a perforated muzzle brake attached on 1/2 x 28 threads making the overall length 18 inches; the bore is six groove right hand 1:8.5 FN twist. The sight...