As the bullet passes the gas port, some of the expanding gas travels through the gasblock into the gas tube.ģ From here it flows into the gas key which directs the gas into the expansion chamber in the carrier.Ĥ The expanding gas exerts pressure on the gas rings on the bolt assembly forcing the assembly to move rearward.ĥ As the assembly travels, the cam pin forces the bolt to rotate 15 degrees and unlock from the chamber.Ħ With the extractor grabbing the rim of the spent casing, the brass is pulled from the chamber and in conjunction with the momentum of the BCG, flung through the ejection port.ħ Gas is also vented through 2 holes on the side of the BCG. We are going to briefly cover a simplified description of how the BCG functions.ġ When the trigger is pulled, the hammer hits the firing pin sending it traveling forward through a hole in the bolt assembly to strike a primer.Ģ When the primer detonates and ignites the powder, the expanding gas propels the bullet down the barrel. Many enthusiasts like to build AR-15 rifles and it is important to know exactly how the firearm works. Key components of an AR15 bolt carrier group include the carrier, bolt assembly (with extractor), gas key, cam pin, firing pin retainer pin, and the firing pin. Understanding which BCG is right for you will involve an examination of your intended use, performance requirements, budget and desired aesthetics. At Primary Arms, we carry more than 100 different BCGs. Choosing a BCG can be overwhelming given the multitude of choices on the market. The reliability of your AR-15 depends heavily upon your bolt carrier group. With each shot the BCG violently travels to the rear of the rifle into the buffer tube where it is stopped and returned by the buffer and buffer spring.Įven in a semi-automatic firearm, this process can happen multiple times per second depending on the skill of the shooter. The BCG is subject to extreme pressures and temperatures as it harnesses the gas from the Direct Impingement (DI) gas system to cycle the rifle. It loads a new round into the chamber, fires that round, extracts and ejects the spent casing, cocks the hammer and repeats the process. The Bolt Carrier Group (BCG) is a critical component of the AR-15.
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