Product Description The Rock River Arms A4 mid-length complete upper receiver is built to deliver hard-hitting performance in .450 Bushmaster while maintaining the handling and modularity of the AR platform. It features a 16-inch chrome moly barrel with a 1:24 twist, a mid-length gas system with a low-profile gas block, and an RRA Operator Brake with 5/8-32 threads to help manage recoil. A 13-inch M-LOK compatible lightweight free-float rail provides a rigid mounting platform for accessories. This upper arrives ready to install with a bolt carrier group, charging handle assembly, and one 5-round magazine. Product Highlights Chambered in .450 Bushmaster for powerful hunting and heavy-impact applications 16-inch chrome moly barrel with 1:24 twist for dependable durability and performance Mid-length gas system with low-profile gas block for a streamlined build 13-inch lightweight free-float M-LOK rail for accessory mounting and improved handling Complete upper includes bolt carrier group, charging handle assembly, and one 5-round magazine Specs Brand: Rock River Arms Manufacturer Part Number: 450B0592 UPC: 842834125548 Platform: A4 complete upper receiver Caliber: .450 Bushmaster Barrel Length: 16 in Barrel Material: Chrome moly Twist Rate: 1:24 Gas System Length: Mid-length Gas Block: Low profile Handguard: RRA 13 in extended lightweight free-float rail (M-LOK compatible) Muzzle Device: RRA Operator Brake Muzzle Threads: 5/8-32 Included: Bolt carrier group, charging handle assembly Magazine Included: 1 magazine Magazine Capacity: 5 rounds Disclaimer: We strive to provide accurate and up-to-date product descriptions, specifications, and images; however, errors or omissions may occasionally occur. We are not responsible for any inaccuracies or discrepancies in the content provided. If you notice anything incorrect or unclear, please contact us so we can correct it. When in doubt, we recommend verifying details directly with the product&';s manufacturer.