Sir,
The upper is intended to be used in conjunction with AR-pattern lower receiver assembly build with standard AR-pattern components. There are a wide variety of manufacturers of AR-pattern rifles, individual components, and accessories. Among these manufacturers, quality and adherence to an AR-pattern "standard" varies. Our upper has been tested extensively with a variety of lowers built by different manufacturers. However, we cannot ensure that the upper will function with every lower receiver assembly that is available or has been built. Essentially, the SR-556 upper can be installed on any well made AR15 lower or vice versa. Thank you for your email.
