The bolt carrier group, or BCG, is a large component of an AR-15 rifle. It is responsible for the movement of the bolt and for cocking the hammer. The BCG consists of four main parts: the carrier, the key, the gas tube, and the bolt.
The carrier is a cylindrical piece of metal that houses the other three components. The key extends from the carrier and fits into a slot in the gas tube. The gas tube runs from the front of the key to the back of the carrier. The bolt sits inside of the carrier and is locked into place by two locking lugs.
When you pull on a charging handle, it causes movement in all four of these parts. The carrier moves back, which cocks the hammer and unlocks the bolt. The key moves with the carrier and actuates the gas tube. The gas tube vents gas from the barrel into the key, which in turn pushes on the carrier. This action cycles the bolt and loads a new round into the chamber.