This lower receiver is marked 6.8 SPC The engraving designating the caliber is the only difference between this lower receiver and our standard AR-15 lower receiver. The Tactical Machining AR-15 lower receivers are built from mil-spec 7075-T6 forgings from the lead forge in the U.S. They contour is fully machined leaving no flash marks from the forging. We take extra care when machining our lower receivers resulting in tolerances that are tighter and fits that are better than other lower receivers Note: These are considered a firearm and are required by federal law to be shipped to an Federal Firearm License (FFL) holder near you. If you require help finding an FFL holder please do not hesitate to contact us.