Those who like to take personal defense safety seriously know the importance of proper maintenance of their firearms. Something as simple as checking your internals and replacing inexpensive parts in your bolt carrier group can go a long way in keeping your AR-10 running reliably.
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What’s a Bolt Carrier Group?
You can find the bolt carrier group in any semi-automatic rifle or pistol. It acts as the primary action mechanism of a firearm. It also houses the bolt and resets your firearm’s hammer.
The bolt carrier group is an indispensable part of your AR-10. Located in the upper receiver, it’s absolutely integral at enabling your AR-10 to function. Additionally, it is the primary part responsible for your gun’s semi-automatic fire.
Parts of a Bolt Carrier Group
Here are the primary components that make up each bolt carrier group:
It’s very easy to mix up the terms “bolt carrier” and “bolt carrier group.” When we talk about the bolt carrier group, we’re talking about all the functioning parts as a whole. However, the bolt carrier, to simply put, is the housing for the other pieces of the BCG.
The bolt carrier usually features a hardened 8620 or 9310 steel build. It also absorbs the force of the pressurized gas from each fired round and resets the hammer as it goes back and forth within your AR-10 upper.
The carrier key, also known as the gas key, is a protrusion located at the top of the bolt. It’s hollowed out and fits over the end of the Gas Tube within the upper receiver. After firing a round, the carrier key receives the pressurized gas from the barrel chamber through the gas tube. It uses this force to cycle the rifle’s action through the BCG.
Gas rings have a very simple but incredibly important function. They’re like the engines of the car and create the driving force of the BCG. These metal rings have a design that can catch the gases released after firing a round. Then, it uses the force from the pressurized gas to help the bolt unlock and push the backward when cycling.
Bolt and Extractor
The bolt and extractor are gear-faced metal units faced units located at the front of the bolt carrier group. It is also the driving force behind loading new cartridges and ejecting old shells from the magazine to the chamber. During the rearward motion of the BCG, the extractor latches onto the cartridge’s rim and extracts spent cartridge shells from the chamber pulling towards the ejector. The bolt strips a cartridge from the magazine during the forward motion and injects it into the chamber.
When pulling the trigger, the firing pin strikes the primer, causing it to explode. The spark from this process is what causes the gunpowder inside the cartridge to ignite, creating pressurized, super-heated, rapidly expanding gas that propels the bullet forward and cycles the rifle.
The firing pin spans the length of the bolt with the pin head towards the front.
Also known as the Bolt Cam, this tiny BCG part plays a big role in keeping the shooter safe when firing. As pressurized gas enters the carrier key on its way to the gas chamber it triggers the Cam pin. During the rearward motion, the cam pin forces the bolt to rotate inside the bolt carrier, unlocking the bolt from the barrel chamber and allowing the rifle to cycle.
During the forward motion, the cam pin rotates again, this time locking the bolt to the barrel chamber. This creates a tight-lipped seal that protects the shooter and the weapon from bolt blowback. A force of 50,000-60,000 psi super heated pressurized gas, normally contained in the barrel chamber, explodes towards the BCG and the shooter.
How Does the Bolt Carrier Group Work?
Each time you fire your AR-10, some of the pressurized gas from the spent round is sent down the gas tube to the carrier key of the bolt carrier. The pressurized force from the spent cartridge fills up the gas chamber towards the tail of the bolt sending the BCG backwards. This rearward motion triggers the Bolt Cam in the BCG, unlocking the bolt from the rifle chamber, and allowing the entire apparatus to rearwards, compressing your rifle’s buffer spring.
As the bolt carrier travels to the rear, an extractor hooked on to the rim of the cartridge, pulls the spent casing into the ejection port removing the spent casing from the gun entirely. The hammer is cocked on the rearward stroke of the BCG marking the first half of the BCG’s action cycle.
Once you eject the magazine, the compressed buffer spring sends the entire BCG forward. As the BCG moves back to its starting position, the Bolt strips a cartridge from the magazine and chambers the round. Afterward, it locks it into the barrel chamber, creating a tight-lipped gas seal.
Without a properly functioning bolt carrier group, your AR-10 will cease to function as a semi-automatic rifle. As a result, you need to manually cock your AR after every shot. One of the most common stoppage issues experienced by AR owners, a failure to extract, is directly related to issues on the BCG as well.
Why Upgrade Your BCG?
The bolt carrier group is the part exposed to a lot of wear and tear. Each time you fire a round from your AR-10, the BCG has to withstand friction from involuntary movement, immense heat from gas pressure, spring compression, and other physical forces.
With any mechanical action, repeated movements and stress will inevitably wear down the part of your BCG. The springs, in particular, tend to suffer the worst, with repeated compression, heat, and cooling eventually weakening if not snapping the spring altogether.
The good news is that with proper cleaning, parts maintenance, and quality upgrades, you can prevent most rifle stoppage issues and you can also minimize the wear and tear of your BCG. Bolt carrier group parts and upgrades are relatively inexpensive, easy to install, and will only take a few minutes out of your day to replace.
Buyer’s Guide to the Best AR-10 Bolt Carrier Groups
If you’re only looking to replace an individual part of the carrier group, we also have different guides to help you out.
Look for a bolt carrier group that passed MPI Testing. Magnetic Particle Inspection (MPI) testing is a quick and easy testing method to determine near-surface or surface issues on ferromagnetic, high-quality metals. It’s a great way to detect cracks, seams, laps or inclusion in your BCG that can create further issues later on or even potentially damage your weapon or injure you. Since high quality, BCGs use durable steel components, your BCG manufacturer shouldn’t be skimping out on this quality control method.
The metals used can affect the durability and weight of your BCG. Most BCGs you’ll find in the market will most likely have a steel construction. More specifically, they have a military grade steel build. For years, this was the standard metal used for military firearms.
Nowadays, aluminum and titanium are starting to gain popularity and are metals you’ll probably run into when looking into BCGs. Although both significantly more expensive than steel, they make great lightweight alternatives for low-mass BCGs often used in tunable gas systems or competition rifles.
You might be able to find lightweight versions of mil-spec steel BCG but chances are they’ll have a lot of lightning cuts, a process or removing large sections of metal to reduce weight. This might give you a cheaper lightweight steel BCG but you’ll also be sacrificing some durability.
Manufacturers add finishes to their BCG to enhance durability. However, a good quality finish will also improve inherent lubricity. Additionally, it would require less maintenance and add a nice look to your BCG. Also, consider how often you’ll be using your AR-10 before deciding on an expensive BCG finish that you might not even need.
There are 4 common finishes that you’ll most likely encounter when looking for a new BCG. Phosphate finishes are the common military standards. However, they are rough and will require a lot of lubricant. Nitride finishes dark in color, smoother and better for wear and tear, but costs more.
Nickel boron finishes are considered premium finishes, so expect to break your budget on nickel boron finished BCGs. A nickel boron finished BCG will have a stainless finish, requires very little lubricant, and is significantly more durable than standard phosphate finishes.
Finally, Diamond Like Carbon (DLC) finishes are the creme-de-la-creme of finishes. It’s smooth and extremely durable so expect it to last a long time with very little required maintenance outside of standard cleaning. It’s also one of the most expensive finishes on the market, but the quality justifies the price.
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The 5 Best AR-10 Bolt Carrier Groups
If you’re a competitive shooter, this bolt carrier group by JP Enterprises will give you that competitive edge you need to outperform your competition. Machined out of tough, corrosion-resistant stainless steel, the JP Enterprises .308 LMOS BCGs features a design that can minimize the impulse of the rifle and cool down fast after extended firing sessions.Â
This BCG is lightweight, weighing just under 9 ounces. It’s light enough that you should feel a noticeable weight reduction when carrying and shooting your AR-10. This BCG also comes with wide carrier rails with an extra bearing surface that improves bold alignment and reduces wear. It also comes with the complete JP EnahncedBolt system designed to handle extreme heat and high-impact use.Â
Additionally, it comes finished in both a Melonite QPQ and chromium nitride coating, making this one of the most rust-resistant and surface wear-resistant bolt carrier groups that you can get for your AR-10.
- Superior, lightweight construction
- Perfect for rifles with tunable gas systems
- Corrosion-resistant and cools down quickly
- Ideal for competitive shooting
- Needs an ajustable gas block for better recoil
- Only for small frames
CMMG .308 Bolt Carrier Group: Best value pick.
CMMG built the CMMG.308 Bolt Carrier Group to handle the high duty applications necessary during tactical operations. Consequently, it’s one of the most securely made bolt carrier groups that you can purchase. It features a build manufactured from 8620 steel with a chrome-lined interior and an extractor fashioned from S7 tool steel.
Although it looks parkerized, all its internal and external parts are coated with phosphate. Phosphate finishes are often seen on military weaponry and equipment because they provide excellent corrosion resistance and are relatively easy to use. However, phosphate finishes leave so you might want to shoot a few magazines through your AR-10 to break it in.
- Phosphate finish for good rust and corrosion resistance
- Optimized for the .308 Winchester round
- Built to handle high duty tactical applications
- Constructed from durable 8620 steel
- Will need to be broken in
Brownells .308 AR Bolt Carrier Group: Best budget pick.
If you’re looking for a high-quality, multi-purpose BCG then consider Brownells .308 AR Bolt Carrier Group. This BCG is machined from shot peened, heat treated 9310 steel making it both tough and durable. It features 8620 steel to make the actual bolt carrier with torqued screws. The extractor also has a rubber insert and spring for better long-term durability and performance.
This Brownells .308 AR Bolt Carrier Group is compatible with just about every other caliber of the AR-10, including the 6.5 Creedmoor, .260 Remington, .338 Federal, .243 Winchester, and 7mm. It’s also MPI inspected for surface flaws to prevent possible rifle stoppages and premature BCG failure.
For added durability and longevity, Brownells uses a black nitride coating for upgraded rust, corrosion and surface wear resistance. It also comes in a Nickel Boron coating if you’re looking for that extra mileage and protection or if you feel like spoiling yourself a little.
- Will function properly with a miriad of calibers for the AR-10
- Comes in a Black Nitride or Nickel boron coating
- Great for both beginners and experts
- Compatible with DPMS style rifles
- Reasonably priced
- No inner chrome platings
- Not high pressure tested
- Not compatible with armalite-style uppers
You can identify this BCG immediately with its distinct laser-engraved Aero Precision ‘A’ emblem. It also features forward assist serrations and a chrome-coated firing pin.
When it comes to Aero Precision, durability and performance are expected. This bolt carrier group is no exception as it’s MPI/HPT tested and made of 8620 steel. Every part has a black nitride coating to reduce wear and tear and ensure reliable performance for a long time. Ready to go right out of the box, his bolt carrier group from Aero Precision is the ideal firing mechanism for a big-bore rifle construction.
- Sleek, distinct look and black nitride finish
- Built for long-term durability
- Straightforward to use
- Forward assist serrations
- May not fit some firearms
The Faxon Firearms .308 9310 Bolt Carrier Complete is a superb choice for competitive shooters searching for accuracy and functionality. This high-performing bolt carrier group is constructed from shot peened 9310 steel ensuring long-term durability and performance.
This Faxon Firearms .308 bolt carrier group has chambered bolt lugs for increased dependability. This BCG comes in a black nitride finish for improved weathering and corrosion resistance. It’s also nice how highly polished everything is, which helps decrease fouling and friction, reducing maintenance while giving you a smoother, upgraded performance.
The only issue with this gun is the current extractor has been known to fail leaving the spent casing inside the chamber. However, Faxon firearms has acknowledged this issue and has addressed it with their manufacturer. Faxon Firearms BCG owners can be rest assured that the failed extractor is covered with Faxon Firearm’s warranty and replacement part will be sent to you free of charge.
- Shot peened 9310 Steel Construction with Enhanced bolt assemblies
- Great for competitive shooting
- Extremely polished finish decreases friction and fouling
- May need to replace the extractor
The Brownells .308 AR Bolt Carrier Group is a popular choice in the AR community for a reason. This is a solid BCG pick, designed to function well with nearly every caliber the AR-10 is made for. It’s great for shooters of every skill level and comes in two durable, wear-resistant finishes. It’s also a great multi-purpose pick that comes at a good price point. Just make sure you have the proper DPMS style rifle since this BCG won’t work with Armalite style uppers.
How to Install a Bolt Carrier Group for the AR-10
These are the 6 simple steps you need to follow to install your new BCG at home (assuming the old BCG has been removed):
- Drop down the charging handle into the channel located within the upper.
- Take the bolt carrier group, and align the gas key with the gas channel that is located inside the charging handle.
- Push the entire bolt carrier group forward towards the receiver. Eventually you’ll hear an audible click letting you know that the bolt carrier group is in lock position.
- Assemble the upper and the lower receiver together.
- Confirm that your bolt carrier works by pulling back the charging handle multiple times.
- If the charging handle works, you can conduct a live test.
Suggested read: Our guide on the best AR-10 barrel recommendations. Definitely worth checking out if you are starting a new build.
Any one of the bolt carrier groups that we’ve covered here today will make a solid upgrade whether you’re building an AR-10 or simply replacing a stock BCG. If you do end up going a different route, you can always follow our buyer’s guide section that we went over earlier to help you find a good lower receiver as well.
A few reminders before you go. There are a lot of large rounds made for the AR-10 but not every bolt carrier group is compatible with every cartridge or built to handle the most powerful AR-10 rounds. Also, every rifle upper will be compatible with every BCG, as with the Brownells .308 AR Bolt Carrier Group. So remember, it’s your responsibility to get a BCG that will work for your vision, usage, and set-up.