An assembled bolt carrier group (BCG) is the heart of the gas-operated cycling mechanism in AR-15 style rifles and similar firearms, enabling semi-automatic or automatic firing. The assembly includes several key components: the bolt carrier, which serves as the main housing and movement mechanism; the bolt itself, which locks into the barrel extension to fire and unlocks to eject spent casings; the firing pin, which strikes the primer to initiate firing; the cam pin, which allows the bolt to rotate and unlock; and the gas key, securely attached to the carrier top, which catches gas from the barrel and uses it to push the carrier back, cycling the action. This complete assembly is crucial for the firearm’s reliability, as it ensures smooth operation, feeds new rounds into the chamber, and expels spent casings, all in a rapid and coordinated sequence. Proper maintenance and periodic inspection of the BCG are vital to preserve its functionality and safety.