Cornerstone; "the chief foundation on which something is constructed or developed" When you want to start your AR build, the lower is the starting point. The fit and finish of Grey Ghost Precision's cornerstone will prove to you that you have the right beginning to a reliable and strong AR build. When coupled with their Grey Ghost Precision uppers, the match is very hard to beat. Characteristics: Machined to tighter than Mil-Spec tolerance, the critical tooling dimensions are held to within +/ - .001" Machined from forged 7075-T6 Aluminum Black Hard anodized with a surface hardness of 60 Rockwell Features a nylon tipped tensioning screw Mfg: Grey Ghost Precision