AR57 ULT Upper Receiver Assembly 12in Barrel 5.7x28mm Caliber. 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 Device: Muzzle Brake Finish: Melonite Color: Black Fabric/Material: 7075-T6 Aircraft Grade Aluminum 8620 Alloy Steel Gun Type: Rifle Automatic Rifle Gun Make: AR Platform Included Accessories: Standard Condition: New Magazine Included: No Rifling Profile: 6 Groove Rifling Mount Type: Picatinny Attachment Type: Picatinny Package Contents: AR57 ULT 12in Barrel 5.7x28mm Upper Receiver Assembly AR57 AR57ULT-12: ULT Upper Receiver Assembly 12in Barrel 5.7x28mm Caliber