AR57 ULT Upper Receiver Assembly 12in Barrel 5.7x28mm Caliber AR57ULT-12. 7x28mm Upper Receiver Assembly: Manufacturer: AR57 Caliber: 5.7x28mm Overall Length: 24.5 in Barrel Length: 12 in Barrel Twist Rate: 1-8.5 Threads per Inch: 1/2x28 Muzzle...