AR/M4 Bolt Carrier Group. Which one?

  The bolt carrier group, or “BCG”, is 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 goal of this article is to tell you what features and characteristics you should look for in a BCG, including a breakdown of the many coating types available.

What to Look for in a Bolt Carrier Group

AR-15 BCG vs. M16 BCG

You will sometimes see both AR-15 and M16 varieties of BCG’s, also known as “semi auto” and “full auto” varieties. The difference is in the carrier itself. On the M16 BCG, there’s a little more material on the back of the carrier, which allows compatibility with a full auto sear. This means that the M16 BCG weighs a bit more than the AR-15 BCG.So which one should you get? We recommend the M16 style. The added weight can help the rifle cycle more smoothly – especially on a carbine. Plus these are much more common.One more thing to note: a M16 BCG is not considered a machinegun part by the BATFE, and there are no “special regulations” that apply to a M16 BCG vs the AR-15 BCG. This was backed up in an official statement from the BATFE:“An M16 bolt carrier is simply a machinegun part and as such its domestic sale and possession is unregulated under the Federal firearms laws. It is not unlawful to utilize a M16 machinegun bolt carrier in a semiautomatic AR15 type rifle.”

MPI and HPT Tested Bolts

Due to the extreme forces exerted on the bolt, it’s critical that no defects exist in the metal. The best way to assure this is through MPI and HPT testing. HPT = “high pressure testing”, while MPI means “magnetic particle inspected”. Per military specifications, the manufacturer should mark the bolt after the tests are completed.

Chrome lined gas key and carrier

Most carriers and gas keys are chrome lined. This mil-spec requirement makes the part easier to clean and function more reliably when dirty. Note: this is different than a chrome coating on the outside of the carrier – chrome lining refers to the inside surfaces.

Bolt Material & Shot Peening

The most common steel used in the bolt is Carpenter 158 steel. Some manufactures are using 9310 steel. Which is better? It depends who you ask, but most will agree that the difference in steel is small, and both varieties of steel work just fine.Many manufacturers also “shot peen” their bolts. This is a process that essentially makes the metal more resistant to cracks. During the shot peening process, tiny spheres known as “shot” are blasted at the bolt, with the shot acting as a tiny peening hammer that relieves stress from the metal.

Proper staking of the gas key

With every shot, high pressure gases blast through the gas tube, then into the gas key on the BCG. The gas is attached with two small hex screws, and with time, the forces of pressure can cause the screws to loosen. To prevent this, the screw heads should be “staked” – which means that some of the metal on the gas key is smashed into the screw heads, so they can no longer move.

Lightweight Bolt Carrier Groups

Lightweight bolt carrier groups can remove as much as 8oz of weight, perfect for lightweight AR builds. There is a catch – this will change how your weapon cycles, so you will need an adjustable gas system to properly regulate the flow of gas to the BCG. This is most commonly accomplished with an adjustable gas block, but other options exist.

Bolt Carrier Group Coating Options

Manganese Phosphate Coating (Parkerized™ Coating)

This is the mil-spec coating for AR-15 BCG, and by far the most common coating you will see. It’s economical, durable, and highly corrosion resistance. Since the surface is more porous than some of the more exotic coatings, it’s harder to clean. This may not be the most exciting BCG coating, but with over 50 years of service in the US military, you can’t go wrong with phosphate.

Chrome Coating

Chrome possess superior hardness and reduced porosity vs a phosphate BCG, making smooth and easy to clean. There are still chrome BCG’s on the market, but they have lost some popularity with the rise of newer coating technologies. The big downsides of chrome are that is expensive, and the finish can become brittle and flake off with time.

Nitride Coating (Nitrocarburizing, Black nitride, Melonite™, QPQ, Tennifer™)

Nitride finish, commonly known by the brand names of Melonite™ or Tennifer™, isn’t technically a coating. It’s a chemical treatment that results in a hardened black surface that is both durable and highly corrosion resistant.The coating involves high temperatures of nitrocarburizing that can have a negative effect on the sensitive parts in the BCG when done improperly, so choose wisely. But when the process is done correctly, you’ll have a very tough, no-frills BCG that is easier to clean than phosphate coatings.

Electroless Nickel Coatings (NiB, EXO™, NP3™, Nickel Boron Nitride)

The electroless nickel coating process results in a thin surface layer that is harder than the underlying metal, with excellent dry lubricant properties. This finish comes in many varieties, with the most common being nickel boron (NiB). Dozens of other variants exist, such as Nickel Boron Nitride, EXO™, and NP3™. These varieties tend to enhance the hardness and/or lubricity of the coating.Electroless nickel coats are easy to clean and less reliant on lubrication. So what’s the downside? The biggest issue is that the electroless nickel coating is known to wear off with time. This affect can be reduced by sticking to high quality parts and enhanced coating varieties, but it is still worth noting.

Vapor Deposition Coatings (IonBond™, Titanium Nitride/TiN, PVD, CVD)

Commonly referred to as IonBond™, CVD, and PVD coatings, this category of coatings is characterized by specific manufacturing process known as vapor deposition. The resulting coating is thin, extremely hard, and slippery - reducing the need for lubricant and speeding up cleaning. This is newer technology, but the favorable characteristics of vapor deposition coatings have made this the fastest-growing category of BCG coatings. As an added bonus, the manufacturer’s ability to customize the color of the coating has led to some exotic new BCG colors hitting the market.The big downside with this type of coating is cost – often 2 or 3 times the price of a phosphate BCG.Technical Notes:Ionbond™ is technically a brand-name. “PVD” and “CVD” Physical Vapor Deposition and Chemical Vapor Deposition – two similar but distinct coating processes. CVD is generally viewed as superior to PVD – it provides a more complete coating that adheres better to the metal below. The actual material used in the TVD and CVD coating can vary - the most common is Titanium Nitride (TiN) however many other varieties exist, with varying colors and physical characteristics.

Choosing a Bolt Carrier Group

For most builds, a mil-spec, phosphate coated BCG will be just fine. Always make sure that your bolt is shot peened and MPI/HPT tested, and the gas key is properly staked.If you’re interested in upgraded coatings, we are big fans of the EXO and NP3 coatings, as well as PVD/CVD coatings. Black nitride and chrome can also be good, but stick to reputable sources. We would steer clear of cheap NiB coatings – they are known to have issues.

Looking to buy a BCG for your next build?

AT3 Bolt Carrier Groups deliver the best value for top-tier mil-spec M16 BCG's. 

AT3 Tactical partnered with the best BCG manufacturer in the industry - Toolcraft Inc - to produce top-tier bolt carrier groups with excellent prices. The BCG's are available in both Mil-Spec Phosphate and Black Nitride finishes. 

They feature MPI/HPT inspected bolts, properly staked grade 8 gas key screws, and an unconditional lifetime warranty. 

Actual Military Manual for M16/M4

army_M16_m4_service_manual (pdf)