AR-15 BCG: The Bolt Carrier Group

Project ar15 Sep 25, 2023
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Ar15 Bolt Carrier Group

Well known as the "Heart" of the Ar15, the Bolt Carrier Group or BCG is a critical component of the platform. 

As briefly mentioned in our article AR15: General Specifications and Terminology (Jargon) ( bolt carrier group is a part of the Ar15 that reciprocates within the upper receiver. It is responsible for stripping, chambering and extraction of cartridges and fired casings.  When reciprocated by charging handle or discharge of a cartridge, the carrier also resets the hammer as it moves rearward for subsequent firing.

What is The Bolt Carrier Group?

The Bolt. The bolt makes contact with the barrel extension and rotates to "lock" into place when in battery. The bolt also contains the extractor and extractor spring. The extractor "grabs" each cartridge when one is stripped from the magazine and facilitates the extraction of that spent casing when fired. 

Gas rings. These thin metal washer-shaped parts on the bolt ensure the proper amount of gas passes into the carrier system to ensure rotation and "unlocking" of the bolt from the barrel extension and reciprocation of the BCG. Due to their exposure to high pressures from fired ammunition, bolts need to be manufactured to a very high standard. This includes being high pressure (HP) tested and magnetic particle inspected (MPI) for structural integrity and micro fracturing in the metal. 

The Firing Pin. The firing pin is shaped like a long thumb tac. And goes through the bolt. When the trigger is pulled it releases the hammer and the hammer strikes the firing pin which subsequently strikes the primer of a chambered cartridge, thereby discharging it. 

The Carrier. This is the housing that contains all the aforementioned parts together as a "group" as indicated by its name. BCGs come in different finishes, to aid in overall reciprocation and functionality of the part within the upper. Some popular finishes include: Nitride, Phosphate and Nickle-Boron.

Carrier Key: This part is attached to the top of the carrier and directly receives the gas and pressure from the gas tube when the gun is fired. It's shape and connection to the carrier body are directly responsible for the transfer of the gas and pressure to the kinetic energy of the Bolt carrier group.  

Ar15 bolt carrier group parts diagram

What Are The Two Types of Bolt Carriers?

There are two models of carriers. Ar15 and Full Auto. The two are distinguished by their shape at the bottom rear of the carrier. This is simply an indication of its compatibility with an ar15 that has the components necessary to fire fully automatic. Ar15s that are full auto capable, require a full auto BCG to do so. However, AR15s that are not capable of fully automatic fire can utilize either AR15 carriers or full auto carriers. So just to be clear: a Full Auto Carrier WILL NOT make your semiautomatic Ar15 fire full auto. 

Ar15 bolt carrier group full auto carrier

Field stripping and cleaning your Ar15 regularly is critical to identifying failing parts or unusual wear. Thoroughly inspect your bolt carrier group while cleaning and apply proper lubrication before use. The Bolt Carrier Group is a critical group of components to the Ar15 platform. And is there for often the subject of much consideration by Ar15 owners. Again, thanks to the Mil-Spec standard, it's easy to get a bolt carrier group that runs great at an affordable price. And of course, many others to upgrade to.