A Buyer's Guide to the Bolt Carrier Group

Many describe the bolt carrier group, or BCG, as the heart of an AR-15 rifle. It consists of about half-a-dozen parts working together to discharge a round and cycle the action. It’s also a very technical gun part that fans of the AR-pattern rifle like to nerd out over. For those reasons, this article is written for the intermediate AR shooter—those who think they might want to upgrade their rifle but need a little more information. In this article, we’ll cover what a bolt carrier group is, how it works, and the different materials it’s constructed from. 

What Is a Bolt Carrier Group?

The BCG as we know it was designed by Eugene Stoner, the gun maker who invented the AR-15 for the gun company ArmaLite. With the AR platform, Stoner wanted to create a rifle better than the AK. He believed he could do that with a system that operated along the same axis. The concept here is that the rifle would be more accurate because the mechanical components and propellant gases all move in a straight line. Keeping that fundamental concept in mind, it’s easier to understand how an AR-15 works and why Stoner designed it that way. 

What Does a Bolt Carrier Group Do?

When you pull the trigger on an AR rifle, the hammer inside the receiver pops up and strikes the firing pin, which pierces the primer on the cartridge in the chamber. A tiny explosion burns up the gunpowder inside the casing and creates gas. That gas pushes the bullet out of the barrel, but then some of it is captured by a gas tube at the end of the barrel. The gas tube directs the gas to a gas key fixed on the bolt carrier. That gas drives the BCG back. 

When the AR15 BCG moves rearward, a few things happen. First, the bolt rotates and unlocks from the chamber. As it pulls back, the extractor grabs the spent casing, which gets ejected from the chamber. Second, the BCG pushes the hammer back to reset. As the BCG moves, it presses against the buffer and compresses the buffer spring. Lastly, the spring bounces the BCG forward, so it strips a round from the magazine and pushes it into the chamber. Once all the way forward, the bolt assembly locks in place, and the rifle’s ready to fire again. 

What Parts Make Up a Bolt Carrier Group?

Although we just went over how an AR-15 bolt carrier group works, we also want to identify the main parts of a BCG so that you can identify them when you field strip your rifle. These include: 

• Carrier - the main body that holds all the pieces together
• Bolt Assembly - this fits into the end of the carrier and holds the gas rings and extractor. Its purpose is to lock into the chamber, guide the firing pin, and extract the casing. 
• Bolt Cam Pin - a large but short pin that controls the movement of the bolt. It guides it forward and back. 
• Firing Pin - the part that sets off the chain reaction by piercing the primer in the cartridge. It fits into the bolt assembly. 
• Firing Retaining Pin - a tiny pin that inserts into a tiny hole and holds the firing pin in place
• Gas Key - a tube fixed to the carrier by two screws. It catches gas from the gas tube. 

Bolt Carrier Group Materials

BCGs are available in a variety of metals and finishes, but understanding the benefits is no easy task. While there are back-and-forth arguments about the best bolt carrier group configurations, a lot of it is based on specific performance expectations. 

Metals

Most manufacturers build BCGs out of steel. More specifically, Mil-Spec steel, which has become a fairly broad standard for gun parts. Other common metals include aluminum and titanium. Because they’re both lightweight, they’re used to build low-mass carriers, which are popular for competition rifles or rifles with tunable gas systems. While they both cost more than steel, titanium costs the most.

While material is an obvious way to reduce weight in the bolt carrier, so are lightening cuts. This is when the manufacturer removes large sections of metal from the carrier. The thought here is instead of using expensive lightweight metal, just use less metal. Such designs are, again, usually used for low-mass carriers. 

Finishes

Manufacturers finish their products to extend the life of the construction material. For gun stuff, the finish has some type of inherent lubricity and increased durability. In general, better quality finishes result in BCGs that are more durable and easier to clean. But they’re also more expensive, so before you buy, it’s important to consider how hard you’re going to run your gun.

OpticsPlanet offers nearly 50 different finishes when it comes to BCGs, so we’re not going to list them all in this article. However, we will highlight some of the most common ones. 

These include:

• Phosphate - This is the Mil-Spec standard, so it’s durable and corrosion-resistant. But it’s a rougher finish, so you would have to use more lubricant and clean it more often. 
• Nitride - This is considered a notch above phosphate, meaning it has a smoother finish. However, it costs more. Also, like phosphate, it’s dark in color.
• Nickel Boron - This is where you start getting into more premium finishes. A nickel boron BCG usually has a stainless finish, and it’s very slick and durable. Again, it costs more.
• DLC - Diamond Like Carbon is the creme-de-la-creme of finishes. It’s considered extremely durable and smooth.

The overall benefit of buying an AR bolt carrier group with a high-quality finish isn’t necessarily to improve your overall performance but rather, to extend the life of the part. 

Parting Shots

If you’re considering upgrading your AR15 bolt carrier group, it’s easy to get lost in the details. While you can’t go wrong with an expensive titanium DLC, you can’t go wrong with an economical Mil-Spec steel nitride either. It depends on how much you plan on shooting and under what conditions. 

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