AR 15 - Bolt Carrier Groups and Parts

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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.

Is your rifle a budget build? Defensive carbine? Hunting rifle? Work horse utility rifle? Lightweight race gun? Competition shooter? This buyer’s guide will explain the various types of bolt carrier groups, coatings and what differentiates one from another.

An AR-15 bolt carrier group consists of the following components as identified in the photo below; the carrier, bolt assembly which includes the extractor, gas key, cam pin, firing pin retainer pin, 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.

- When the trigger is pulled, the hammer hits the firing pin (1) sending it traveling forward through a hole in the bolt assembly (3) to strike a primer.

- 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.

- From here it flows into the gas key (7) which directs the gas into the expansion chamber in the carrier (2).

- The expanding gas exerts pressure on the gas rings (6) on the bolt assembly forcing the assembly to move rearward.

- As the assembly travels, the cam pin (8) forces the bolt to rotate 15° and unlock from the chamber.

- With the extractor (5) 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.

- 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.

- 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.

- 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 (4) will push a bullet up a feed ramp into the chamber.

- 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.

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

Now that you know how the BCG works, its 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 and the options have become confusing.

One of the beauties of the AR-15 platform is the ability to switch calibers by swapping a complete upper or replacing the barrel, bolt and magazine on your rifle. This article will focus on BCG types and coatings,not calibers. Most of the same options discussed are applicable to the numerous calibers you can build as direct impingement ARs.

The first choice you must make is what type of BCG to install. There are essentially 3 types of AR-15 BCGs:

- M16 (full-auto rated).

- AR-15 (semi-auto rated).

- And low mass/lightweight.

There are some subtle and not so subtle differences amongst the types. For simplicity sake, the design differences come down to mass and weight as pictured below.

The M16 BCG is the design used by the military and is rated for full-auto fire. It is sometimes referred to as a full-auto BCG, but do not be alarmed that by itself does not make an AR into a machine gun and it is not a regulated part. It is heavier by design to slow the rate of fire when automatic or burst is selected on a military M16.

All mil-spec BCG’s will be of the M16 type and it is the most common BCG type on the market. Mil-spec BCG’s weigh on average 11.6oz and require a 3-5oz buffer to cycle properly.

In the AR-15 enthusiast community there is a strong segment that wants everything on their AR-15 to be mil-spec. The design is reliable, has been combat proven for decades and is clearly the most popular choice.

For the overwhelming majority of AR-15 owners, their rifle is not full-auto capable. It is not necessary to have that extra weight to slow down the firing rate when your rifle is semi-auto.

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 is the low-mass or lightweight BCG that has removed much of the body of the carrier and/or uses lighter alloys than the Mil-Spec machined 8620 steel. Low-mass BCGs are easy to identify as most are skeletonized by eliminating unnecessary materials.

The lightest on the market have been reduced to the 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. This weight savings comes at a cost, however, low-mass bolt carrier groups are more expensive than Mil-Spec and 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.

It is recommended to use an adjustable gas block to fine tune your gas system. It is also recommended to use 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.


The M16 bolt carrier group is the most popular choice on the market. You seldom have to worry about gas system changes or reliability issues.

For the beginner enthusiast, the M16 type is highly recommended. It is also recommended for any type of duty work or if this is the shooter’s only or primary AR-15 where reliability is an absolute must.

The AR-15 type BCG is an acceptable alternative for those that want to reduce a little weight from their rifle without sacrificing reliability.

Low mass/lightweight BCGs are recommended for more advanced enthusiasts that have more than one AR-15 and want to increase performance and cut down on weight as much as possible. They are ideal for competition shooters where any improvement in performance gives 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 a mindboggling number of coating names that you can find on a BCG. The biggest argument that you get from Mil-Spec proponents is that phosphate gets the job done. Its good enough for the military and it’s all you need.

Technology and innovation, however, have advanced significantly since the early days of the Vietnam War when the M16 was introduced.

Advanced coatings today are measured by Rockwell hardness scales, lubricity, and friction coefficients.

All claim to be superior than the Mil-Spec phosphate finish. Not only can advanced coatings improve the performance and durability of your BCG, they can make cleaning a breeze. As Wynn Atterbury, President of WMD Guns, a leading manufacturer of firearms and nickel boron BCGs says, “Our mission is to optimize a user’s shooting experience. We want to make shooters happy and make shooting more enjoyable.

Our coatings increase reliability, shooters get longer usage between stoppages, slicker operation and make cleaning easier.”

Manganese Phosphate Coating

Military rifle BCG’s use a manganese phosphate coating, also known as Parkerized™ or simply phosphate coating.

It is a comparatively less expensive 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.

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 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.

It is recommended if you are budget-conscious or want a Mil-Spec rifle. No frills, it just works.

Nitride Coating

The nitride coating process is a high temperature nitrocarburizing chemical treatment that creates a smooth, hardened black surface. It results in high durability and is highly corrosion resistant.

The process has many slight variations and is also known as 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.

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


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

Electroless Nickel Coatings

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.

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


Electroless nickel coated bolt carrier groups are easy to clean and provide a smoother action for your BCG.

They are recommended for shooters that have a little more budget and want to upgrade their shooting and maintenance experience. Nickel boron BCGs are also popular with shooters that want to add some color to their black rifle.

Ceramic Top Coating

For shooters that want the benefits of a nickel boron BCG, but do not want a shiny metal part on their rifle, a ceramic top coat over nickel boron may be a good option. Ceramic top coating is the process of applying a thin layer of ceramic paint (CereKote or DuraCote) to a coated bolt carrier group.

Unlike other coatings, the ceramic top coat will wear off quickly at the points where the BCG makes contact with the receiver (these points are only visible when the BCG is out of the rifle for maintenance).

When applied in conjunction with a nickel boron coating, it exposes the undercoat allowing the shooter to experience the benefits of the reduced friction on the action.

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


Ceramic top coats are only recommended for shooters that want to add a cosmetic element of color to their rifle.

They are most often used in conjunction with a nickel boron coating where the shooter wants the benefits of higher lubricity, but does not like the shiny metal.

Physical Vapor Deposition Coatings

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, which means that 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-15’s like gold, silver and bronze.

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


Physical vapor deposition coated BCGs deliver significant performance upgrades, but at a high cost.

They are recommended for competitive shooters that need every additional edge possible when it comes to performance. For shooters that have the budget, PVD coated BCGs are an attractive upgrade for your rifle if you like the performance enhancements and unique look.

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.