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AR-15 Bolt Carrier Group Frequent Asked Questions

The Bolt Carrier Group (BCG) is a critical component of the AR-15. The BCG is the “action” of 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 BCG is subject to extreme pressures and temperatures as it harnesses the gas from the Direct Impingement (DI) gas system to cycle the rifle. 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.

Even in a semi-automatic firearm, this process can happen multiple times per second depending on the skill of the shooter.

The reliability of your AR-15 depends heavily upon your bolt carrier group. Choosing a BCG can be overwhelming given the multitude of choices on the market. At Primary Arms, we carry more than 100 different BCGs. Understanding which BCG is right for you will involve an examination of your intended use, performance requirements, budget and desired aesthetics.

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.

Many enthusiasts like to build AR-15 rifles and it is important to know exactly how the firearm works. We are going to briefly cover a simplified description of how the BCG functions.

1 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.

2 When the primer detonates and ignites the powder, the expanding gas propels the bullet down the barrel. As the bullet passes the gas port, some of the expanding gas travels through the gasblock into the gas tube.

3 From here it flows into the gas key which directs the gas into the expansion chamber in the carrier.

4 The expanding gas exerts pressure on the gas rings on the bolt assembly forcing the assembly to move rearward.

5 As the assembly travels, the cam pin forces the bolt to rotate 15° and unlock from the chamber.

6 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.

7 Gas is also vented through 2 holes on the side of the BCG. The moving carrier cocks the hammer preparing the fire control group for the next shot.

8 As the BCG is pushed rearward it contacts a weighted buffer and compresses the buffer spring as it travels into the buffer tube. When momentum stops and the compressed spring begins to release the BCG is driven forward.

9 If the magazine is empty, the BCG will be engaged by the bolt catch and stop forward motion to allow for a magazine change. If the magazine is not empty, the bolt head will push a bullet up a feed ramp into the chamber.

10 When the round is chambered, the iconic looking bolt head is rotated 15° and locked into the barrel extension. This enables the bolt to withstand the extreme pressures produced by firing a round.

11 The process repeats after the next pull of the trigger.

Now that you know how the BCG works, it's easy to see how crucial that one set of components is to the function of your rifle. The U.S. Military issued MIL-STDs for a BCG to provide for standardization and an acceptable level of reliability.

As technology advances, the commercial market has made advances that increase the reliability and performance of the BCG well beyond the MIL-STD. The reality today is that not all BCGs are created equal and which one you choose for your rifle is subject to a variety of factors.

One benefit of the AR-15 platform is the ability to switch calibers by swapping a complete upper or by replacing the barrel, bolt, and magazine. When building a rifle, you’ll need to choose a BCG style that makes sense for the rifle’s purpose.

There are essentially 3 types of AR-15 BCGs:

- M16 (full-auto rated)

- AR-15 (semi-auto rated)

- Low mass/lightweight

There are some subtle and not-so-subtle differences between them, but for simplicity’s sake, the major differences come down to mass and weight.

The M16 BCG is the design used by the military and is rated for full-auto fire. Some companies will call them ‘Full Auto BCGs’, but don’t confuse them for an auto-sear: these BCGs do not make your rifle a machine gun. Basically, M16 BCGs are slightly heavier to slow the rate of fire for automatic or burst fire. On average, M16 BCGs weigh 11.6oz and require a 3-5oz buffer to cycle properly.

All MIL-SPEC BCGs will be M16-style, making it the most common BCG type on the market. The design is reliable, and with decades of proven combat effectiveness, M16 BCGs have earned their favor with American gun-owners.

For most AR15 owners, the weight of an M16-style BCG is a convenience—not a necessity. Without select-fire, a standard AR15 BCG will offer similar performance for most rifles. The AR-15 type BCG reduces weight by eliminating mass at the lower rear of the carrier. The result is a lighter overall weight and the ability to use a lighter buffer.

Taking the weight reduction concept to an extreme, low-mass or lightweight BCGs remove much of the body of the carrier and/or use lighter alloys than the Mil-Spec machined 8620 steel. Low-mass BCGs are easy to identify, as many feature a skeletonized design that sheds any ‘excess’ material.

The lightest BCGs on the market weigh in 5oz range, cutting more than half the weight of a Mil-Spec BCG. An additional benefit of a lighter BCG is a faster cycling AR with less felt recoil.

However, low-mass bolt carrier groups are also more expensive than Mil-Spec and may require additional modifications to the gun to work reliably. The Mil-Spec DI gas system is intended to operate a certain weight BCG. When you reduce that weight, the gas pressure may become too strong and cause reliability issues.

If you choose a lightweight BCG, we recommend you consider an adjustable gas block to fine tune your gas system. We also recommend purchasing lighter buffers and, in some cases, lighter springs. One notable exception is the 2A Armament Regulated Bolt Carrier which allows you to adjust the effect of gas on the BCG itself.

All-in-all, low mass/lightweight BCGs are recommended for more advanced enthusiasts that have more than one AR-15 and want a specific type of performance. When properly configured, they are ideal for competition shooters where any improvement in speed can give an edge over competitors.

In the early 1960’s, some original M-16s had chrome-plated bolt carrier groups; however, the military soon transitioned to a more economical and longer-lasting phosphate finish, which has endured for decades. Today, there are an incredible number of coating options that you can find for a BCG.

Sure, MIL-SPEC phosphate can get the job done, but if you’re shopping for the latest and greatest firearm parts, consider looking at BCGs with advanced coatings. Technology and innovation have advanced significantly since the Vietnam War, and modern BCG coatings can easily outperform the traditional phosphates. Modern BCG coatings are stronger, more reliable, and easier to clean—and in such a competitive market, prices aren’t too bad either.

Military BCGs use a manganese phosphate coating, also known as ‘Parkerizing’.

Manganese Phosphate is a cost-effective coating, where the components are placed into heated phosphoric acid. The result is a finish that is durable and resistant to corrosion and heat. The combination of low cost, high durability, and military use make phosphate-coated BCGs the most popular coating on the market.

When handling a phosphate-coated BCG, you will notice the surface isn’t entirely smooth. This causes friction as the action operates, which requires larger amounts of lubrication for the firearm to function reliably.

More lubrication also means that the components will attract more dirt and debris. The rough surface also makes cleaning difficult and time consuming. Phosphate coated BCGs need regular cleaning maintenance to function reliably. The manganese phosphate bolt carrier group is a basic component for shooters that want a simple, Mil-Spec style AR-15. Backed by over 50 years of combat-proven service in the military, it has become the most popular BCG coating and it gets the job done effectively.

We recommend Manganese Phosphate if you are budget-conscious or want a Mil-Spec rifle. No frills: it just works.

1Most economical coating on the marketHigh coefficient of friction, requires more lubricant to operate smoothly
2Durable and corrosion resistantBCG must be disassembled and cleaned regularly
3Trusted by the militaryRough surface texture makes cleaning more difficult

The nitride coating process is a high temperature nitrocarburizing chemical treatment that creates a smooth, hardened black surface. Nitride coatings have a high durability and corrosion resistance.

The process has many slight variations, including Salt Bath Nitriding, Melonite, Tennifer, Quench- Polish-Quench (QPQ), Black Nitride, and others. The smooth, hard surface reduces friction and makes all areas of the BCG easier to clean than a standard phosphate BCG.

Nitride coated bolt carrier groups are an upgrade over the basic phosphate coating. They are great for shooters that want an easier cleaning experience, smoother action, and the look of the black nitride finish.

1Upgrade from phosphate without a considerable inscrease in priceSlight price increase from phosphate BCG
2Durable and corrosion resistantBCG must be disassembled and cleaned regularly
3Easier to clean than a phosphate BCGProcess must be done correctly or it can result in brittle components, stick to well-known brands
4Requires slightly less lubricant than a phosphate BCG

Electroless nickel coating is a chemical process where a layer of nickel metal alloy, most commonly nickel boron, is deposited onto all surface components of a bolt carrier group.

Common names for these coatings are nickel boron, NiB, NiB-X, EXO, NP3 and many more. Electroless nickel coatings are very slick with a low coefficient of friction. They require minimal lubricants and are very easy to clean. Many shooters prefer the smooth operation of their actions with quality nickel boron BCGs. Because the coating is nickel, the surface appearance is shiny metal and may not be suitable for those that like an all-black AR-15.

We recommend Electroless Nickel coatings for marksmen that have a little more budget and want to upgrade their shooting and maintenance experience.

1Slick surface with high lubricity and low coefficient of frictionMore expensive than phosphate or nitride BCGs
2Requires minimal lubricationProcess must be done correctly or it can result in poor performance, stick to wellknown brands
3Durable and corrosion resistantShiny nickel appearance (some love it, some hate it)
4Easy to clean
5Shiny nickel appearance

Ceramic top coating is an aesthetic coating applied over a BCG’s existing coat. Think of it as a secondary option to supplement your BCG’s main coating.

Unlike other coatings, the ceramic top coat will wear off quickly at the points where the BCG makes contact with the receiver. On a BCG with a nickel boron coating, this wear exposes the undercoat, allowing the shooter to experience the benefits of reduced friction.

Ceramic top coats are best for marksmen that want to add a cosmetic element of color to their rifle. They are most often used with a nickel boron coating, where the user wants the benefits of higher lubricity, but does not like the shiny metal.

1Multiple color optionsCeramic top coat wears off quickly on contact points
2Dual layer protection when used in combination with an advanced coatingNot very durable

The Physical Vapor Deposition (PVD) coating process uses a vacuum, high temperatures, and evaporation, or sputtering, to create an extremely thin, hard, and slick coating on BCG surfaces.

The process is also known as titanium nitride (TiN), Ion Bond, and CVD. PVD coatings are among the lowest coefficients of friction available.

The surfaces are very slippery and require minimal lubrication. Physical vapor deposition coatings are difficult to apply and require considerable capital expenditures, which considerably increases the cost of these BCGs.

The coating process is also line-of-sight, so complex components like a BCG will have slightly varying degrees of coating thickness in internal areas. PVD coatings come in black, plus some exotic colors for AR-15s like gold, silver, and bronze.

Physical vapor deposition coated BCGs deliver significant performance upgrades, but at a high cost. We recommend PVD for competitors that need every additional edge possible for performance. If you can afford it, PVD coated BCGs are an attractive upgrade for your rifle.

1Very slippery surface with high lubricity and low coefficient of frictionConsiderably more expensive than phosphate or nitride BCGs
2Requires minimal lubrication
3Extremely durable and corrosion resistant
4Easy to clean
5Multiple colors options

When you combine the different types and coatings on bolt carrier groups, your field of choices expands considerably. Ultimately it comes down to your preference. A no-frills Mil-Spec BCG will get the job done.

How much improvement you want to make in performance and appearance is limited only by your budget. While not a requirement for operation in your AR-15, more advanced coatings definitely make cleaning easier, increase reliability, and can add some style to your black rifle.

The Bolt Carrier Group (BCG) is the heart of your AR 15. Buy an AR 15, M16, or low mass complete BCG or build one from a variety of parts available from the world’s top manufacturers. Choose from specialized coatings such as Nickel Boron, Nitride, Melonite or Phosphate. Many Manufacturers use cutting-edge proprietary coatings and make some of the best BCGs on the market.

Now that you know everything about your next Bolt Carrier Group, browse our vast selection of BCGs here at Primary Arms, your one-stop-shop online gun store. Here you will find industry-leading customer service and fast shipping.

If you have any questions, don't hesitate to send us a message at info@primaryarms.com or call us at 713-344-9600.