What is a full auto bolt carrier group?

A full-auto bolt carrier group (BCG) is a component of an AR-15 rifle that allows the weapon to fire in fully automatic mode. The BCG contains a metal piece called the “carrier” that slides back and forth inside the receiver, and a metal piece called the “bolt” that moves forward and backward inside the carrier. When the trigger is pulled, it causes the bolt to move forward, stripping a round from the magazine and chambering it. When the round is fired, the gas pressure created by the explosion pushes on the piston head, which in turn pushes on the carrier, causing it to move rearward. This movement extracts the spent casing from the chamber and ejects it from the weapon. As soon as the carrier reaches the rear of its travel, a spring inside the receiver pushes it back forward, chambering another round from the magazine. This process continues as long as the trigger is held down and there are rounds in the magazine.

Bolt carrier groups come in two different varieties: full-auto and semi-auto. The main difference between the two is the shape of the carrier. The carrier on a full auto BCG has a larger “shelf” on the underside, which extends further back than the shelf on a semi-auto BCG. This shelf engages with the sear, which is what holds the Bolt Carrier Group in the rear position when you release the trigger. Without this engagement, the Bolt Carrier Group would move forward and chamber another round as soon as the previous round was fired, resulting in a fully automatic weapon. The other difference is that full auto BCGs typically have a heavier carrier, which helps to dampen the recoil of the weapon.