The Receiver is the ideal building block for a long lasting AR15. This 7075 T6 Aluminum forged Mil Spec stripped receiver with M4 feed ramps has NiB-X coating inside and out to provide an exceptionally tough, ultra-smooth surface finish for outstanding wear resistance and lubricity. This upper works with standard carriers but is even smoother with a NiB-X coated bolt carrier and NiB-X charging handles, available separately.