Sons of Liberty Gun Works, Bolt Carrier Group, 223 Remington/556NATO, Manganese Phosphate Finish, Black WARNING: This product can expose you to chemicals including lead, which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov. Full Auto Rated and Profiled Bolt is precision machined from Carpenter Technology No. 158 chrome-nickel alloy, Carrier is machined from AISI 8620 aircraft quality alloy Sons of Liberty Battle Flag logo deep laser engraved Sons of Liberty Gun Works looks at the bolt carrier group as the heart of the gun. If the gas port is the lungs, then the bolt carrier group is the heart. It's what drives the entire gun. If you look at the AR platform's cycle of operations, everything happens around the bolt carrier group. Locking, unlocking, extracting, ejectingall of it begins with the bolt carrier group.