Best Bolt Carrier Groups (BCG) For Your AR-15

Originally posted here.

Selecting the right Bolt Carrier Group (BCG) for your AR-15 is more than just a matter of preference; it’s a crucial decision that impacts your rifle’s reliability, performance, and durability.

The BCG acts as the heart of the AR-15, responsible for the seamless execution of critical actions: chambering rounds, igniting primers, ejecting spent casings, and preparing the rifle for the next shot.

It endures extreme conditions, from rapid heating and cooling to intense mechanical stress, underscoring the importance of choosing a high-quality BCG. Material science and engineering innovations have led to various BCG types, including mil-spec, low-mass, and those featuring advanced coatings for reduced friction and increased wear resistance.

The material composition, such as Carpenter 158 or 9310 steel, and the choice of coating, from phosphate to Diamond-Like Carbon (DLC), can significantly affect the BCG’s performance. Whether you’re building a new rifle or upgrading an existing one, understanding the implications of these choices is key to optimizing your AR-15’s functionality and longevity.

We will cover the background information you need to know and explain why Faxon Firearms is our choice for the best bolt carrier group.

Quick Takeaway

• Faxon 5.56 Gunner Lightweight Bolt Carrier Group
• Faxon 5.56 M16 Bolt Carrier Group
• Aero Precision Nickel Boron BCG
• Colt Mil-Spec BCG
• Brownells Nitride M16 BCG
• BCM Mil-Spec BCG

What are bolt carrier groups?

The AR-15 bolt carrier group (BCG) is one of the most important parts of an AR-15 platform. The BCG is responsible for cycling the semiautomatic operation when the trigger is pulled, with a series of steps in this order:

1. Allowing the firing pin to strike the primer on the chambered round.
2. Grabbing and ejecting the spent shell casing.
3. Re-setting the hammer for the next round.
4. Pushing a new round from the magazine and chambering it.

These four actions are automatically performed when the trigger is pulled. The steps of chambering a new round and resetting the hammer allow the semiauto cycle to repeat, eliminating the need for a manual bolt action.

What Makes Up a Bolt Carrier Group?

Carrier

This is the external housing for the bolt and is attached to the gas key. The carrier contains the bolt itself and all its components. The carrier also makes contact with the buffer and buffer spring assembly during the firing cycle and also takes the brunt of the force created by the expanding gases inside the AR-15 platform. Because of this, 9310 Tool Steel is commonly used during the manufacturing process.

Gas Key

Also called the "carrier key", is an essential part as it directs the gas from the gas tube into the bolt carrier. It should be tight and staked to the bolt carrier to create a seal. That seal is important as the gas is what forces the bolt to rotate, locking and unlocking the bolt carrier from the chamber.

Bolt

The AR15 bolt helps guide the round into the chamber, extracts the spent case, then ejects the spent cartridge once you’ve fired the gun. The bolt features a “star" at the front, shaped like a cog, which allows the BCG to rotate 15 degrees to lock and unlock inside the AR-15’s chamber. The bolt has its own set of components; bolt body, gas rings, lugs, extractor, ejector, and the bolt face.

Bolt Gas Rings

The bolt gas rings are located towards the rear of the bolt within the bolt carrier group. Their purpose is simple; they capture the gases and help regulate the pressure to help the bolt lock and unlock during cycling.

Firing Pin and Retaining Pin

This is pretty self-explanatory. A firing pin is a component that strikes the primer of a cartridge, causing the whole process to begin. The retaining pin keeps it…well….retained.

Cam Pin

The cam pin keeps the bolt inside the bolt carrier and keeps it in line, it keeps the firing pin in line, it "cams" the bolt into the locked position as the BCG finishes its forward movement into battery. The cam pin is inserted into the bolt carrier through a hole offset from the top. The cam pin also has a hole through the middle of it for the firing pin to slide through. Having the firing pin move through this hole keeps everything moving and rotating as one component.

If you’ve disassembled your bolt carrier group and noticed excessively worn components, now is the time to replace those. CMMG has a killer bolt rehab parts kit for just this reason.

If you really want to keep it simple, then just keep a spare bolt handy. I know of several professional firearms instructors and active-duty special operations guys, that just keep a spare bolt so they don’t have to worry about individual parts when time is of the essence.

FAXON 5.56 AR15 BOLT, NITRIDED

What is the difference between AR-15 and M16 bolt carrier group?

Installing an M16 BCG in your AR-15 won’t convert it to full auto. That would require swapping out the trigger, disconnector, hammer, and safety selector lever. Full-auto BCGs and commercial BCGs perform exactly the same and they use the same parts.

The only real noticeable difference and some won’t even notice, are the physical dimensions of the carrier itself.

The slightly larger rear section of the M16 BCG allows it to work with an auto sear. The auto sear in an M16 works by allowing the full-auto bolt to lock and unlock continuously while the trigger remains depressed.

The auto sear needs that extra steel on the bottom of the M16 bolt to properly function. Without it, the auto sear can’t be “tripped", thus allowing the hammer to cock and de-cock (firing rounds automatically) without pulling the trigger every time.

In order for all this to happen, you need to have a lower receiver that is specifically designed for full auto functionality. Commercial lower receivers are not fabricated in a way to allow for the full-auto parts to be installed.

Is using an M16 bolt carrier group illegal?

It is NOT illegal to use an M16 BCG in your AR-15. The M16 bolt carrier groups themselves are not regulated by any federal laws or ATF regulations. However, if you were to get your hands on a full-auto sear and full auto lower receiver parts kit, you’re going to have some explaining to do when the ATF comes knocking.

Is Nitride Good for BCG?

Nitride, also known as Melonite or Salt Bath Nitride (SBN), is a heat-treating process that creates a hardened surface by diffusing nitrogen into the metal surface. The resulting metal surface is extremely smooth and hard which makes it highly resistant to wear and corrosion whether this is your home defense AR-15 or your professional duty rifle.

The primary benefit of the nitride heat-treating process is that it does not require chrome lining of the gas key or internal areas of the bolt carrier itself, keeping manufacturing costs down. Additionally, the smooth finish requires less lubricating oil and is easier to clean.

What is Magnetic Particle Inspection Method?

Bolt carrier groups are subjected to immense amounts of pressure, heat, and friction. The pressure is somewhere on the magnitude of tens of thousands of pounds per square inch.

If any part of the bolt carrier group components contain microscopic cracks created during the manufacturing process, then using that BCG could result in a catastrophic failure to the firearm and possibly the shooter. To ensure this doesn’t happen, all Faxon bolt carrier groups undergo a magnetic particle inspection.

During an MPI test, the BCG is placed inside a magnetic field, usually created with two electromagnets. A liquid solution containing magnetic particles is then applied to the steel. The magnetic particles will adhere themselves to any cracks or tiny imperfections that are on or just below the surface.

"Magnetic particle Inspection (MPI) is a nondestructive testing (NDT) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source." – Wikipedia

Best Bolt Carrier Groups

A lightweight BCG is designed for a rifle with an adjustable gas block and a builder who’s willing to spend the time tuning the gun. The heavier standard bolt carrier groups, such as the M16 BCG, are better suited to running any kind of ammo through any kind of gas system.

When it comes to either a lightweight BCG for more precision-tuned guns or a BCG that will run just about anything, Faxon has you covered. I like to have a gun that will run regardless of what ammo I throw at it and the outside conditions I’m running it in. That’s why I run the Faxon M16 BCG in my AR-15.

Faxon 5.56 Gunner Lightweight Bolt Carrier Group

When looking at lightweight bolt carrier groups, Faxon was unsatisfied with what the market had to offer.

Seeing a capability gap, They set out to create a bolt carrier group that could live up to the Gunner name with no compromise in strength or function. Faxon set specifications that would allow no compromise.

FAXON 5.56 GUNNER LIGHTWEIGHT BOLT CARRIER GROUP, NITRIDED

By coating all parts, the carrier’s natural slickness is increased markedly requiring less gas and pressure to operate.

Further, the Nitride process seals components against corrosion while making them fantastically wear-resistant. Gas keys use grade 8 fasteners and are fully staked for a lifetime of service.

Faxon 5.56 M16 Bolt Carrier Group

Seeing “MIL-SPEC" as just the starting point, Faxon set out to ensure the best options are available for its demanding customers.

These options include using 9310 tool steel for the bolt, nitriding the assembly to increase surface hardness, decrease resistance, and have superior corrosion resistance compared to typical chrome and phosphate bolt carrier groups.

FAXON 5.56 M16 BOLT CARRIER GROUP, NITRIDED

All M16 bolt carrier groups are fully MPI tested to ensure the material is sound and without defects. Bolt lugs are chamfered 45 degrees to ensure reliable action for even the highest pressure hand loads and fast lightweight actions.

What Are Users Saying About the Faxon Bolt Carrier Groups?

• "Excellent value and quality for the money spent. I have close to 300 flawless rounds on this BCG at the time of this review."
• "Bought with the barrel in a combo set. Headspace was spot on and seems like a quality product. Pleased."
• "Beautiful fit and finish. Zero failures out of 200 plus rounds!! Awesome craftsmanship! Thanks folks!"

Aero Precision Nickel Boron BCG

The Aero Precision Nickel Boron BCG boasts an incredibly smooth and durable finish.

This state-of-the-art coating has been extensively used in the military, including on Army 60mm and 81mm mortar tubes, Blackhawk helicopter hoists, and certain Navy ships for rugged applications.

5.56 BOLT CARRIER GROUP, COMPLETE – NICKEL BORON

Aero Precision provides their Nickel Boron BCGs with or without their logo, and their logos are expertly crafted and visually pleasing. Additionally, they offer non-logo options for those who prefer a more minimalist look.

Colt Mil-Spec BCG

When it comes to mil-spec M-16/AR-15 parts, Colt sets the gold standard. If you’re in search of mil-spec components, you won’t find anything better than Colt.

COLT M16 5.56 BOLT CARRIER GROUP

This particular BCG has all the features mentioned earlier that are part of the mil-spec standard, including a phosphate finish, HPT/MPI bolt, and a chrome-lined carrier and gas key.

Brownells Nitride M16 BCG

If you’re on a tighter budget, the Brownells Nitride M16 BCG might be the right choice for you. The Nitride finish adds a slickness that makes cleaning much easier.

BROWNELLS M16 5.56 BOLT CARRIER GROUP NITRIDE MP

While the BCG is MPI tested and generally mil-spec, it differs from the standard in that its bolt is made of 9310 steel, rather than Carpenter 158.

However, for the average shooter who puts a few thousand rounds downrange per year, this is not going to matter. And although it may not be mil-spec, it’s worth noting that this BCG is about half the price of other big-name brands.

Bravo Company Manufacturing M16 BCG

The Bravo Company Manufacturing (BCM) Bolt Carrier Group (BCG) is designed for AR15, M16, and M4 platforms, emphasizing reliability and quality. Featuring a parkerized exterior and a chrome-lined interior, this auto version BCG is machined to USGI specifications, ensuring optimal performance.

BCM FULL AUTO M16 BOLT CARRIER GROUP

The gas key, critical for proper gas system functioning, is chrome-lined, heat-treated, and secured with Grade 8 fasteners, properly staked per military specifications. The bolt assembly, made from Mil-Spec Carpenter No.158 steel, undergoes a shot peening process to increase strength and includes a tool steel machined extractor and ejector.

Each bolt is High Pressure Tested (HPT) and Magnetic Particle Inspected (MPI) for assured quality. Before shipment, BCM test-fires each BCG to ensure functionality, resulting in handling marks indicative of their stringent quality control procedures.