The AR-15 rifle is one of the most iconic and widely used firearms in the United States. A crucial component of this firearm is the AR-15 Bolt Carrier Group (BCG). In this article, we will delve into the intricacies of the AR-15 BCG, exploring its anatomy, function, and significance in the overall operation of this popular firearm.

Anatomy of the AR-15 Bolt Carrier Group

The AR-15 Bolt Carrier Group is a complex assembly consisting of several critical components, each with a specific role in the firearm’s function. Here’s a breakdown of its main parts:

1. Bolt: The bolt is a cylindrical component with locking lugs at its front end. It is responsible for chambering a round, locking it in place, and extracting the spent casing after firing.

2. Carrier: The carrier is a rectangular piece that houses the bolt. It reciprocates within the upper receiver, cycling through the stages of the firing process. The carrier has a gas key on top, which interfaces with the gas tube to facilitate semi-automatic or automatic fire, depending on the rifle’s configuration.

3. Cam Pin: The cam pin sits inside the bolt and carrier and helps rotate the bolt when it’s locked into the barrel extension. This rotation is necessary for locking and unlocking the bolt during firing.

4. Firing Pin: The firing pin is a slender, pointed rod that protrudes from the rear of the bolt and ignites the primer of the cartridge when struck by the hammer. It plays a vital role in initiating the firing sequence.

5. Extractor: The extractor is a claw-like component on the side of the bolt responsible for gripping and pulling the spent casing out of the chamber during the extraction phase.

6. Extractor Spring and O-Ring: These components provide tension to the extractor, ensuring it maintains a secure hold on the casing. The O-ring also helps seal the chamber during firing to prevent gas leakage.

7. Gas Rings: Located on the exterior of the bolt, these rings help create a gas-tight seal within the bolt carrier, directing expanding gas from the fired round back into the action to cycle the firearm.

Function of the AR-15 Bolt Carrier Group

Understanding how the AR-15 BCG functions is essential for anyone interested in operating or maintaining this firearm. Here’s a simplified overview of its operation:

1. Chambering: When the rifle is loaded, the bolt carrier group is in its forward position. When the trigger is pulled, the hammer strikes the firing pin, which ignites the primer in the chambered cartridge.

2. Unlocking: After ignition, the gas generated from the fired round flows through the gas port in the barrel and enters the gas key on top of the carrier. This gas pressure pushes the carrier rearward, unlocking the bolt from the barrel extension by rotating it counterclockwise.

3. Extraction: As the carrier moves rearward, the extractor grips the spent casing, pulling it out of the chamber.

4. Ejection: Once fully extracted, the spent casing is ejected through the ejection port in the upper receiver.

5. Chambering a New Round: The rearward movement of the carrier also pulls the bolt back, cocking the hammer and preparing the firearm for the next shot. A new round from the magazine is chambered as the carrier returns to its forward position.


The AR-15 Bolt Carrier Group is a critical component of the AR-15 rifle, responsible for the firearm’s reliable operation. Understanding its anatomy and function is essential for firearm enthusiasts, owners, and those interested in the mechanics of this iconic firearm. Proper maintenance and care of the BCG ensure the longevity and performance of the AR-15, making it a safe and effective tool for various shooting applications.

By Grace