A bolt carrier group, or BCG, is a critical component of an AR-15 rifle. It is responsible for housing the bolt and enabling it to cycle smoothly. The bolt carrier group consists of the bolt carrier, the gas key, and the firing pin. The bolt carrier is a heavy-duty carrier that houses the bolt and ensures it stays properly aligned as it moves back and forth within the rifle. The gas key sits atop the bolt carrier and routes gasses from within the barrel to power the action of the firearm. Finally, the firing pin sits within the bolt carrier and strikes the primer of a cartridge to initiate the firing sequence.
BCGs are typically made from either steel or aluminum. Steel is the most durable option and will usually last longer than an aluminum BCG. However, steel BCGs are also much heavier than their aluminum counterparts. Aluminum BCGs are lighter weight and will not last as long as steel BCGs, but they are less expensive.