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Among the different AR-15 accessories, the bolt carrier-group (BCG) is considered the workhorse of the AR-15. It is complex and full of moving parts – enduring extreme temperatures and pressures while reliably cycling round after round.
The BCG controls all functions of the AR15 – chambering, firing, and extracting spent rounds. The majority of weapon failures stem from issues with the bolt carrier group, and a good bolt carrier group can keep the failures to a minimum. The BCG includes the following components;
The most common issues with the BCG assembly are a broken firing pin, loose gas-key, broker/worn extractor, and broken/worn bolt lugs. It is a good idea to either have a replacement firing pin and bolt assembly handy, or an entire BCG. Why ruin the plinking or hunting day over a $59.00 part?
First off, “full-auto” BCGs are legal and don’t magically make your AR-15 automatic firing. The slightly longer and heavier bolt carrier group has an additional lug at the rear of the bolt carrier group.
On a full-auto AR-15 or M16, this rear lug pushes the sear release downwards which allows full auto (or burst) fire. In a semi-automatic rifle, this has zero effect on how the gun functions. Semi-auto carriers are now the ugly step-child of the BCG world and don’t have much purpose.
Mil-Spec is a bit of a buzzword in the AR community. Mil-Spec generally means the rifle is built to pass military specifications. Mil-spec doesn’t mean greatest ever, or worst, it’s just in spec to a military contract. A mil-spec BCG features
The coated BCGs are advantageous in that they cut down on friction, dissipate heat, and create a corrosion proof surface. They require less lubrication, are easy to clean, and last much longer. Only the bolt carrier is coated, not the bolt assembly.
The main component of the BCG is the bolt. The bolt has lugs on the front as well as an extractor. While the lugs are milled into the bolt, the extractor is a separate part that is installed with a spring and pin. The bolt also has gas-rings. While extractors and gas rings can be easily replaced, the bolt will be useless if the lugs become overly worn or broken. Premium bolt lugs usually have been hardened and are more durable than economy bolts.
Not every bolt fits every caliber. The following bolt faces are the most common for the AR15 in rifle calibers. The bolt carrier and other parts are the same.
So, one AR15 bolt carrier with a firing pin can house the 6 different bolts above. The 410 BCG is an altogether different animal and we'll leave it alone here.
The following bolt faces are the most common for the AR15 in pistol calibers. The bolt carrier and other parts are NOT the same as the rifle caliber.
AR10 (DMPS) style BCGs are larger than AR15 BCGs and required a different bolt. The standard AR10 Bolt fits