Importance of the AR15 BCG – Bolt Carrier Group

To say a bolt carrier group (AR15 BCG) is the heart of your AR-15 style rifle is an understatement. Without it, your rifle is just a rather poor club. Having a bolt carrier group manufactured from quality materials is of supreme importance.

The Bolt Carrier Group (BCG) is a critical component of the AR-15 (or M16/M4). 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.

Note: In this article, we are covering bolt carrier groups for standard direct-impingement AR-style firearms. Chances are, your AR-15 is a direct impingement gun, which is to say the gun operates by redirecting some of the gas directly back into the chamber, pushing the BCG back to eject the spent cartridge and load a new round.

Some AR-15 variants operate using a gas-piston system where the redirected gases push a piston, which pushes the BCG – this is similar to how an AK-style rifle operates.

Components Of A Bolt Carrier Group – AR15 BCG


This is the external housing for the bolt and is attached to the gas key. It’s important to clean and check it regularly to ensure that the screws attaching it to the gas key do not come loose, and also for signs of excessive wear, cracks, etc.


The bolt helps guide the round into the chamber, extracts the spent case, then ejects the spent cartridge once you’ve fired the gun.

Gas key

Also called the “bolt carrier key”, this essential part funnels gas from the gas tube into the bolt carrier. It should be tight and staked to create a seal. Proper staking keeps those screws from backing out.

Firing pin

A firing pin is a component that strikes the primer of a cartridge, igniting it and causing a small explosion to propel the projectile forward.

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, and it provides many more functions that are crucial to the operation of the firearm. “Battery” is a firearms term for being ready to fire.

Types Of AR15 Bolt Carrier Groups – AR15 BCGs

When going out to select a nifty new BCG for your AR-15, you’ll be presented with an extensive array of styles. The main types can be broken down into some basic groups though.

M16 Or Full Auto Bolt Carrier Group

“Full auto – but wait, don’t I need to be a manufacturer or jump through all these bureaucratic hoops to have a ‘full auto’ anything?”

In this case – no. The full-auto bolt carrier group for your AR-15 is named as such because it was developed as the BCG for the M16 and M4 rifles. It has more material on the “back” of the carrier in order to trip an auto sear, which is the critical component to enable full-auto operation. Without it, your rifle only functions in semi-auto.

Having a full-auto BCG in a semi-auto only AR is perfectly legal around the country.

As a matter of fact, the full-auto style is now the standard style for BCGs, such as those manufactured by E2 Armory. It’s higher mass ensures smooth operation and more reliable cycling of the weapon.

A full-auto BCG can be considered “mil-spec” or better, provided it is made of the proper material, typically the bolt itself consisting of Carpenter 158 steel, and the carrier being 9310 or 8620 steel, with 9310 being a little more sturdy. For reference, 8620 steel is the material used in bolts destined for the M4A1 rifled used by our armed forces. You’ll be fine with 8620 steel.

Semi-Auto AR-15 Bolt Carrier Groups

AR-15s used to ship with semi-auto-only bolt carrier groups, and some low-end models still do. These BCGs shaved down the “back” of the carrier, denying it the ability to trip an auto sear or lightning link for full-auto operation. Generally, you don’t want this sort of BCG, since they tend to be less reliable than their standard full-auto counterparts.

Thankfully, E2 Armory’s bolt carrier groups are all full-auto style, leaving the guesswork out of the equation.

Low-Mass Bolt Carrier Groups

Some companies pride themselves on making what are called “low-mass“ bolt carrier groups. These BCGs strip out weight wherever possible, while still promising full functionality. They’ll look like cheese graters and you’ll wonder if they work well. Some do, some don’t. Low-mass BCGs are mainly targeted at those who build rifles for competition, or those who play the game of building the lightest AR-15 possible. That being said, a low-mass BCG should never be in a fighting rifle. Stick with known-good components and concepts, such as those offered by E2 Armory.

AR15 BCG Materials and Coatings

Beyond the specific types of AR-15 bolt carrier groups, there’s a world of materials and coatings.


Steel AR-15 Bolt Carrier Groups

Various grades of steel make up the most common AR-15 bolt carrier groups. Typically, the carrier itself is machined from 8620 or 9310 steel, with the bolt itself being Carpenter 158. The firing pin may be 8740 steel and chrome-plated for longevity. The gas key itself may be 4140 steel. The bolt itself is Carpenter 158. Those numbers refer to the composition, usage and hardness of the steel used.

For reference, a bolt-carrier group found in a military-issue M4 from FN or Colt will usually use 8620 in the carrier and Carpenter 158 in the bolt itself.

Titanium Bolt Carrier Groups

Titanium is an incredibly durable and resistant material, as well as being lightweight. It’s also not incredibly common and difficult to work with. An AR 15 bolt carrier group made out of 6AI-4V Grade 5 titanium can run more than double the cost of a comparable steel BCG. However, if you’re in the ounces-turn-into-pounds game, it could be worth it. The advantage of course is that it’s a softer recoil – but if you’ve trained on a “normal” BCG, you probably won’t notice.


There’s a few major types of coatings for AR bolt carrier groups.

Parkerized Manganese Phosphate

This coating is the military-spec one. The AR sitting in your safe right now probably has this coating. It’s matte dark grey/black, resists corrosion, and holds up extremely well under stress. Don’t shy away from it just because it’s “common”. It’s harder to clean since the texture holds fouling, but not by much.

Nickel-Boron (NiB) Coating

Nickel-Boron is an extremely durable and smooth coating, with a low friction quality. If you pick it up, it actually feels slippery all on it’s own. Generally glossy-silver in appearance, the coating is super-easy to clean, especially since you can see the fouling easier. E2 Armory offers a solid-performing, and sharp-looking BCG in this coating.

Black Nitride

Black Nitride renders your BCG in a glossy black finish. It’s actually not a coating, but a full-on steel treatment that hardens the BCG more and imparts an impressive corrosion resistance. It’s also very low-friction.

A Note On Coatings

In your research, you may see claims that BCGs coated a certain way can run “dry”. Truth be told, any BCG can run dry, the specific coating only allows it to do so for a longer period of time. Best practice is to lubricate the BCG as part of normal maintenance, regardless of coating.

Testing A Bolt Carrier Group

Any reputable manufacturer, such as E2 Armory, will state their testing protocol for their bolt carrier groups. The tests determine whether the BCG can hold up under stress and pressure. Your BCG is the “heart” of your rifle – it needs to hold up. Generally, two tests are performed. Magnetic Particle Inspection (MPI) and High Pressure Testing (HPT). A quality BCG will be proof-marked (engraved) with “MPI/HPT” or something similar if it’s been through the process.

Magnetic Particle Inspection

Magnetic Particle Inspection, or MPI, is where the bolt carrier group is magnetized and then temporarily coated with magnetic iron particles, usually themselves coated with a dye that reacts to ultraviolet light. The particles will group themselves in defects and cracks (if any) and glow intensely under the UV light.

High Pressure Testing

High Pressure Testing, or HPT, subjects the BCG to pressures generally in excess of the usual pressure of the firing sequence. This overpressure tests to see if the BCG can hold up to sustained live firing.

E2 Armory’s AR Bolt Carrier Groups Meet and Exceed Military Specifications

In so many ways, a proper AR 15 bolt carrier group is the heart of a truly robust and reliable fighting rifle. E2 Armory is proud to offer several variations of BCG, all manufactured in our state-of-the-art facility which also provides OEM services to manufacturers which fulfill requirements for our armed forces. With E2 Armory’s components all meeting and exceeding military specifications, you can be assured that a bolt carrier group by E2 Armory will be the reliable core component of your AR-15 rifle.

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