The Grey Ghost Precision Upper Receiver is a perfect companion for your GGP Cornerstone Lower receiver. After all, accuracy comes from tight, precise tolerances that minimize all variables from the moment the hammer strikes the firing pin until the round leaves the muzzle. By perfecting the critical tooling dimensions to the smallest degree, a forged receiver can punch the same hole down range. FeaturesMachined to tighter than Mil-Spec tolerances, the critical tooling dimensions are held to within +/ - .001"Machined from forged 7075-T6 AluminumBlack Hard anodized with a surface hardness of 60 RockwellM4 Feedramps.250" Takedown pin holesEngraved T-marksFunctions with nearly all available AR-15 parts kits and lowers