Rock River Arms Complete Upper - Versatile, Reliable, and Ready to Perform Rock River Arms is known for delivering dependable firearms trusted by professionals and enthusiasts alike. Their complete upper receivers provide a simple, effective way to finish your build or switch calibers with ease. Chambered in .300 AAC Blackout, this upper offers smooth performance and versatility across a range of shooting applications. It features a durable free-float handguard with plenty of space for accessories, along with a full-length top rail for mounting optics or sights to match your setup. Ready to attach to any standard AR-15 lower receiver, this upper comes complete with a bolt carrier group and charging handle-giving you a dependable upgrade right out of the box.