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 more expensive ARs 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. We’ll cover that world in a future article.
OK with that out of the way - a bolt carrier group is made up of several components.
- Carrier. 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.
- Bolt. 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 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
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. 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 M4 bolts. 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. But in my experience, you have to work to find those. 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.
Again, the standard is full-auto - but double check when purchasing to be safe.
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 maintaining 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 rilfes for competition, or those who play the game of building the lightest AR-15 possible. If you have the cash and time, indulge yourself. However, my gut tells me I wouldn’t want a low-mass BCG in my defensive rifle.
Materials and Coatings
Beyond the specific types of AR-15 bolt carrier groups, there’s a world of materials and coatings. To keep it basic, we’ll cover the common ones, of course.
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? Confused? Yeah, me too. Those numbers refer to the composition, usage and hardness of the steel used. Generally you are good with the combinations I listed though.
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. Thus, expensive. 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. Get one if you want.
Aluminum Bolt Carrier Groups
While not as expensive as titanium BCGs, aluminum ones still manage to be lighter than traditional steel ones. The general use case for aluminum BCGs is in competition use, where again, ounces turn into pounds as they say. The downside is that aluminum is a softer metal and will wear faster. However, if you are competing, you’re blowing through so much ammo during training and running courses, that an extra $150-$200 on top of that isn’t going to make you cry.
It seems every week that some manufacturer comes up with some new whiz-bang coating for an AR bolt carrier group. This is actually pretty normal since most bolt carrier group manufacturers actually only do the “finishing” of their BCGs. They source the raw components from elsewhere - in reality there’s only a few companies that actually make a bolt carrier group. One such OEM is Tool-craft, for example. A company will source from Tool-craft, put their own proprietary coating on it, laser on their logo, and call it a day.
That being said, 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. Some manufacturers will then use a proprietary color process - which usually does nothing but add color and cost. NiB runs a little pricey, but if you can swing it, go for it.
Black Nitride renders your BCG in a glossy black finish. I’m partial to it, it just looks bad-ass. 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. However, I wouldn’t recommend running a bolt carrier group dry in a rifle you plan on utilizing in a defensive scenario. Keep it lubed up, regardless of the claims of “lubricity”.
Testing A Bolt Carrier Group
Any reputable manufacturer 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 over pressure tests to see if the BCG can hold up to sustained live firing.