The greatest improvement to the AR-15, the X-15 SCU (Side Charging Upper). The billet machined X-15 SCU is a revolutionary leap forward in non-reciprocating side charging design that improves where others have left off. A true drop in system that allows the use of a Mil standard bolt without machine modification. The only change you make is install a special Cam pin into your bolt which is included with the upper. Designed to work with .223/5.56, .300 blackout/whisper, 6.5 Grendel, and 9mm the versatile upper works for all popular calibers and all standard and non-standard AR lowers. Machined and profiled to work with all M4 pattern M4 upper forend rails that are designed for use on the mil spec uppers.