Sons of Liberty Gun Works Bolt Carrier Group, 223 Remington/556NATO, Manganese Phosphate Finish, Black SOLGWBCG556 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.