AR-15 Bolt Carrier Group Guide: Everything You Need To Know

The AR-15 bolt carrier group (BCG) is considered by many to be the heart of the AR-15 rifle. The BCG performs many of the main functions of your rifle including firing, discharge of spent rounds, resetting of the action, and chambering of the next round.

Many enthusiasts of the AR platform can obsess over the BCG. The purpose of this article is to help those who want to know more about how the BCG operates, proper cleaning methods, and the best options for bolt carrier groups on the market.

What is the AR-15 Bolt Carrier Group?

The Bolt Carrier Group (BCG) on an AR-15 is the collection of components of the rifle or pistol that performs the following functions when the trigger is pulled:

• Fires the cartridge.
• Redirects gas produced from firing to perform other functions.
• Ejects the spent cartridge after firing.
• Chambers a new cartridge.

Bolt Carrier Group Parts

The BCG consists of the following parts:

• Bolt Carrier – The main body that holds all the pieces together.
• Bolt Carrier Key – This connects to the gas tube and funnels gas into the bolt carrier.
• Bolt Assembly – This fits into the end of the carrier and holds the gas rings, ejector, and extractor. Its purpose is to lock into the chamber, guide the firing pin, and extract and eject the casing after firing.
• Bolt Cam Pin – A short but large pin that controls the movement of the bolt. It guides it forward then back.
• Firing Pin – The part that fits into the bolt assembly and sets off the firing chain reaction by piercing the primer in the cartridge.
• 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.

AR-15 Origins

AR-15 is the name given to any lightweight, semi-automatic rifle based on the Colt AR-15 design.

The AR-15 was developed by Eugene Stoner and his design team at ArmaLite, a subsidiary of the Fairfield Engine and Airplane Corporation. The AR-15 was a redesign of the AR-10. The AR-10 was chambered in the heavier 7.62x51mm, while the AR-15 is usually chambered in the lighter 5.56MM cartridge.

The AR-10 was originally submitted for a 1956 US military competition to design a new rifle for the infantry. The AR-10 originally lost the competition to the M-14, both chambered in the 7.62x51mm cartridge. The M-14 saw major action on the frontlines of Vietnam where infantry soldiers complained of its recoil when fired in full auto, and the difficulty of carrying enough of the 7.62x51mm ammunition to gain fire superiority over the 7.62x39mm chambered AK-47.

This feedback inspired Stoner and his team to redesign the AR-10 with the lighter 5.56mm cartridge and the new design was named the ArmaLite Rifle 15 (AR-15).

AR-15 Bolt Carrier Group Design

Eugene Stoner’s thought process behind the design of the AR-15 was that he could make a firearm that was better than the AK-47 if he made the mechanical components and the gasses that propel the bullet move in a straight line. When this happens, there is less recoil and less stress on the rest of the firearm. Less stress means more durability and reliability.

The initial design of the AR-15 included a tube to vent gas back into the bolt carrier assembly from burnt powder where it expands in a chamber forcing the bolt open to eject the spent cartridge case. The buffer spring in the buttstock pushes the bolt closed causing it to chamber a new cartridge from the magazine. One disadvantage of the bolt and carrier piston system designed by Stoner is the venting of unburned powder residue into the receiver where it may build up causing malfunctions.

Some AR-15 style rifles use an alternative short-stroke gas piston design borrowed from the AR-18, where a metal rod pushes against the bolt carrier, driven by a piston located just behind the barrel gas port. This piston design keeps the rifle cleaner by not exhausting the gas into the receiver. While both the original Stoner and short-stroke gas piston operating systems produce semi-automatic fire, an alternative un-ported barrel assembly includes a sliding handguard connected to a rod moving the bolt by a pump-action.

The original AR-15 bolt carrier group design features a free-floating firing pin. Some AR-15 models feature a firing pin safety in the bolt to reduce the risk of slam-firing. Such firing pin safeties can obstruct the upper to work with standard AR-15 full-height hammers in the fire control group of the lower.

BCG Function: How Does the AR-15 Bolt Carrier Group Work?

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 AR 15 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.

This happens in the following order:

1. The trigger is pulled, and the hammer strikes the firing pin.
2. The firing pin strikes the primer on the chambered round.
3. The extractor on the bolt grabs and ejects the spent shell casing from the round that was fired.
4. The hammer is re-cocked in the lower receiver to prepare for the next round.
5. A new round is grabbed from the magazine and is chambered.

Bolt Carrier Group Construction: Materials & Finishes

There is a wide range of materials and finishes that are used in AR-15 BCGs to help increase reliability and performance. There are some that are ideal for high-performance or competition applications while others are more budget-friendly for the Saturday sport shooters. The “best” option is the one that is the best fit for your budget and usage.

BCG Materials

Most AR-15 bolt carrier groups are made from a high-quality steel known as mil-spec steel. Mil-spec steel has become the standard for most firearm applications due to its reliability and the high standards it’s forged under. Titanium and aluminum are used in applications where weight is a concern, such as competition shooting or if the rifle has a turnable gas system. Titanium is by far the most superior metal to use, and it’s also the most expensive.

Manufacturers have come up with other ways to reduce weight without the need of expensive metals such as titanium and aluminum. The manufacturers will remove large sections of metal from the BCG called lightening cuts that reduce the overall weight of the rifle without compromising on performance or the integrity of the rifle.

BCG Finishes

Manufacturers will use special finishes to help extend the life of rifle components. These finishes make the rifle easier to clean, more durable, and increase the lubricity of the components overall. These finishes can be expensive, so it’s a good idea to take into consideration what your AR-15 will be used for and how much abuse your rifle will take before deciding to spend the money on costly performance finishes.

There are easily over 50 finishes that you could use on your bolt carrier group. We will cover some of the more common finishes:

• Chrome
• Phosphate
• Black Nitride
• Nickel Boron
• Diamond Like Carbon

Chrome

The first AR-15 bolt carrier groups were plated with hard chrome because that’s how Eugene Stoner designed them.

Chrome is well suited for the punishment that the bolt carrier group takes. Chrome is very wear resistant due to the hardness of the metal. Chrome’s high natural lubricity and corrosion resistance also make it a perfect finish for AR-15 BCGS.

Chrome finishes are more expensive and less prevalent than other bolt carrier group finishes.

Phosphate

Phosphate is the current mil spec for bolt carrier groups in M16 and M4 rifles. Phosphate is less expensive than chrome and it’s not as shiny. Chrome’s mirror finish is less than desirable when wanting to be discreet.

BCGs properly manufactured with a phosphate finish will have a chrome lining inside the gas key and the carrier. None of the other finishes on this list will be chrome lined. Phosphate also creates a rough finish that requires more lubricant to run smoothly and makes cleaning more difficult.

Black Nitride

Black Nitride is a heat-treating process that creates a case-hardened surface by diffusing nitrogen into the metal surface. This creates a surface that is extremely hard and smooth making it resistant to wear, easier to clean, and makes for a smoother operation with less lube.

Black nitride finished bolt carrier groups don’t require additional chrome lining, however, they are still more expensive than phosphate finished BCGS.

Black nitride is also referred to as melonite, salt bath nitride, or just nitride.

Nickel Boron

Nickel boron is a very popular finish for AR-15 bolt carrier groups due to it’s high resistance to wear and corrosion on top of having a very low friction factor. Nickel boron bolt carrier groups have greater resistance to corrosion and greater lubricity than both black nitride and chrome BCGs.

Nickel boron is more expensive than other finishes. Phosphate is inferior to black nitride in lubricity, hardness, and smoothness, however, nitride is more expensive. Nitride and chrome are both exceptional finishes, however, chrome is more expensive and harder to find.

Diamond Like Carbon (DLC)

Ionbond Diamond Like Carbon (DLC) is a metal finish applied through a process called Physical Vapor Deposition (PVD). DLC finishes are extremely durable, corrosion-proof, are excellent at reducing friction, and are considered as the crème de la crème of bolt carrier group finishes.

DLC finishes won’t chip, crack, or degrade giving your reliable function and wear protection for much longer than other finishes. It also has a more discreet look which is better than chrome in applications where stealth is a concern.

The PVD application process is difficult, making DLC one of the more expensive finishes for your AR-15 bolt carrier group.

How to Disassemble and Clean Your AR-15 Bolt Carrier Group

Your bolt carrier group on your AR-15 is where most of the wear occurs on your rifle. The combination of carbon build-up and constant friction can cause malfunctions of your rifle such as jams or failure to fire.

Tools and Parts You’ll Need To Properly Clean Your BCG

There are a few things you’ll need to properly clean your AR-15 bolt carrier group. Here are the tools and parts we use as well as what they’re used for:

• CAT M4 Bolt Carrier Carbon Scraping Tool – The CAT M4 tool is a stainless-steel scraping tool used to remove carbon build up from the bolt, bolt carrier, and firing pin flange.
• 1/16” or 3/32” Drift Punch – The drift punch is needed to remove the firing pin retaining pin and removing the extractor pin.
• Small Brass & Nylon Hammer – This hammer is used to help with removing the extractor pin.
• Cleaning Patches, Q-tips, Toothpicks – These will help clean out carbon build up, the Q-tips and toothpicks are for cleaning in tight space of the BCG.
• Gun Cleaning Solvent – Solvent helps to break down carbon build up making it easier to remove. We recommend Hoppe’s Number 9.
• Gun Oil – A high quality gun oil like Ballistol is needed to lubricate all the parts to prevent rust and make the parts easier for assembly and future disassembly.
• Replacement Parts – Some parts of your BCG may break or degrade overtime. We recommend having extra parts such as firing pin retaining pins, bolt cam pins, extractor O-rings, extractors, and firing pins.

Bolt Carrier Group Disassembly

We will start with the disassembly of the bolt carrier group and then we will walk through how to clean all the parts.

Step 1 – Remove Retaining Pin and Firing Pin

Start by removing the firing pin retaining pin by using the drift punch, or the nose of a 5.56mm cartridge, to pry the pin out. Once you remove the retaining pin, then you can easily remove the firing pin.

Step 2 – Remove the Bolt Cam Pin and Bolt

Pull out the bolt cam pin first. Push the bolt head backwards into the body of the carrier (you’ll see the bolt slides and rotates back and forth inside the carrier), and once the bolt head is fully in its rearmost position, rotate the cam pin 90 degrees. You should be able to cam pin straight out of the bolt carrier body.

Step 3 – Remove and Disassemble the Bolt

Remove the bolt from the bolt carrier body. Take your drift punch and remove the extractor pin. Be careful with the extractor pin as it is under tension from the extractor spring. Keep your thumb on the extractor so the spring does not fly out (it’s easy to lose it). Remove the flat extractor and extractor spring, again, while staying mindful of the tension the spring is under.

Step 4 – Remove the Ejector

While not necessary on a routine teardown, it’s fairly easy to remove. Use your drift punch to remove the tiny roll pin and pull the ejector and its spring forward out of the front of the bolt.

Step 5 – Disassemble the Extractor

The extractor has other parts that need to be removed for proper cleaning and inspection: extractor spring, extractor spring buffer in the extractor spring, and a rubber O-ring or D-ring.

Cleaning and Inspection

Extractor

You will start with the extractor. Look at the groove on the underside of the extractor especially if there is dirt and carbon build up so it can properly remove the spent cartridge during operation.

Use a Q-tip to clean out the extractor cutout in the bolt body. You can add a very light coat of oil to help the retaining pin pivot freely. You can also skip the oil if you’d like since oil will attract dirt and grime.

Bolt

It’s generally a good practice to allow the cleaning solvent to penetrate the carbon build up before you try to remove it by soaking it in solvent for a few minutes. Use your CAT M4 Bolt Carrier Carbon Scraping Tool to remove all the built-up carbon off the skinny end of the bolt. Simply insert the bolt into the tool and turn, the tool will scrap the carbon off the bolt as you turn. Use a Q-tip to clean out the holes for the bolt cam pin and all the locking lugs on the head of the bolt. Use a cleaning patch to ensure the face of the bolt is clean as well.

Firing Pin

Grime will build up on your firing pin and prevent the pin’s free travel inside the bolt. Make sure the firing pin is clean or the gun will not function. You can use the circle and cross shaped portion of the M4 Tool to clean the flange on the firing pin.

DO NOT USE OIL ON THE FIRING PIN! Oil will attract dirt and grime preventing its free travel in the bolt.

Bolt Carrier

The bolt carrier on an AR-15 will build up a considerable amount of grime and carbon build up. Just like the bolt, it’s best to let the carrier soak in solvent to allow the carbon to break up a little before cleaning. Inset the skinny end of your M4 Tool into the carrier and twist. The CAT M4 Tool will scrape the carbon from the inside of the carrier as you twist.

Some people will heat up both the bolt and carrier in the oven to open the pores of the metal up, use a light coat of high-quality penetrating lube oil on the inside and outside of these components, and allow the metal to cool so the pores close-up. Remove the excess oil since any oil residue leftover will attract dirt, sand, and grime. Leave just enough to prevent wear and corrosion.

Carrier Key

Inspect the carrier key to ensure that it is snug to the bolt carrier, tighten the screws holding it in place. Take a Q-tip with solvent and insert it into the carrier key to remove any carbon build up.

Gas Rings

These do not need to be removed unless your rifle is malfunctioning or not operating properly. These should be 3 metal rings stacked on one another. Make sure that the gaps in the rings are staggered to create a proper gas seal.

BCG Reassembly

Reassembly of the bolt carrier group for the AR-15 is essentially the reverse of disassembly:

1. Re-install the ejector, spring, and its pin.
2. Then the extractor and its spring, buffer, and O-ring.
3. Put the bolt back into the bolt carrier, making sure the extractor will be facing the ejection port of the rifle once it is installed.
4. Insert the bolt cam pin, rotate it 90 degrees so it will clear under the gas key, and then slide the firing pin into the back of the bolt. Make sure the firing pin slides back and forth freely.
5. Install the firing pin retaining pin into its recess in the side of the bolt carrier.
6. Reinstall the bolt and charging handle in your rifle, and make sure everything functions as it should.

Now that your BCG is clean and reassembled, you’re ready to fire!

Conclusion

The bolt carrier group of the AR-15 is the heart of your rifle. Its unique design makes for a more durable, easier-to-use rifle you can depend on whether you’re hunting, in a firefight, or just plinking at the range. There are many finishes you can choose for your BCG, the one you pick will depend on your budget and your shooting needs.

Take care of your BCG and it will take care of you when you need it the most.

Justin Trump

Justin Trump is the managing editor and owner of CAT Outdoors. The son of a Vietnam veteran, he’s an avid gun enthusiast and 2A advocate. He holds two firearm patents for the CAT M4 and Talon tools. When not managing CAT Outdoors, he enjoys spending time with his family and friends, rooting for Michigan sports teams, and serving his church.