What is the Bolt Carrier Group on my AR-15?

The AR-15 bolt carrier group (AKA the BCG) is the part of the rifle or pistol that is responsible for performing semi-automatic fire when the trigger is pulled. It does this by first allowing the firing pin to strike the primer on the chambered round. Then grabs and ejects the spent shell casing from the fired round. It then re-cocks the hammer in the lower receiver for the next round which ends up grabbing a new round from the magazine and chambering it.

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