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4 Best AR15 Bolt Carrier Groups

4 Best AR15 Bolt Carrier Groups

The bolt carrier group is the heart of your AR15. Let's look at some of the best!

There are two main kinds of AR15 bolt carrier groups - semi auto and full auto.

A full auto BCG does not turn your AR15 into a machine gun. In fact, most AR15s sold today actually come with a "full auto" BCG and there are no legal restrictions against using a full auto BCG.

The only difference between the two is the shape of the carrier. On a full auto carrier the rear lug is larger so it can trip the auto sear on a full auto rifle. It has no function in a semi auto AR15.

One benefit of using a full auto BCG is that it will usually weigh about 1 oz. more than a semi auto BCG. This extra weight can sometimes have a positive effect on reliability, depending on the configuration of your rifle. It is the equivalent of going to a heavier buffer i.e. from a carbine buffer to an H buffer, or an H buffer to an H2 buffer.

The bolt carrier group is the heart of your AR15, meaning a high-quality and reliable BCG is a must-have within your firearm.

We'll look at what makes a good bolt carrier group and our top recommendations, but first, let's hit on the basics.

Nickel Boron BCG Disassembled

Nickel Boron is an upgraded finish on this 5.56 BCG

What is a Bolt Carrier Group?

Before we get into our recommendations, let's do a general overview of how a BCG works within an AR-15. Understanding the bolt carrier parts and how they interact is important to diagnose any issues and ensure your gun has proper reliability and feeding.

A BCG works with the gas system to cycle and eject ammo within your rifle. More specifically, the bolt carrier group will load a new round into your chamber, extract the spent casing, and repeat the process all over again.

Below are some of the key pieces you'll find within a complete bolt carrier group and the general role they play in the process:

  • Firing pin - After the trigger is pulled, a hammer will be released up, hitting your firing pin. Your pin will then strike your round and fire your gun.
  • Carrier / gas key - Your gas key sits on the top of the BCG and connects to the gas tube. When a round is fired through your barrel, gas is funneled back through the gas key and forced down into the body of your bolt carrier group.
  • Bolt carrier - The bolt carrier is the body of your BCG. The high-velocity gas will fill the chamber and force your BCG back into the buffer tube, compressing the buffer and buffer spring. To handle the high temperatures and pressure levels, you'll find bolt carriers and bolts are made from three types of forged steel (Carpenter no. 158, 8620, or 9310).
  • Bolt - The bolt assembly features teeth at the front of your BCG that rotate to lock and unlock itself inside the chamber. As the bolt carrier drives backward, the cam pin is activated, which twists the bolt, unlocking it from the chamber and allowing the BCG to cycle back.
  • Bolt gas rings - These three rings sit on the back of the bolt to create a seal between the bolt and the bolt carrier, trapping gas expansion inside the BCG.
  • Extractor - As the bolt rotates, the extractor hooks onto the rim of your round, pulling it out of the chamber. The spring forces the spent casing out of the upper receiver. As the bolt carrier and bolt return forward, the extractor will pick up another round.

Semi Auto vs Full Auto BCG

There are two main kinds of AR15 bolt carrier groups - semi auto and full auto.

A full auto BCG does not turn your AR15 into a machine gun. In fact, most AR15s sold today actually come with a "full auto" BCG and there are no legal restrictions against using a full auto BCG.

The only difference between the two is the shape of the carrier. On a full auto carrier the rear lug is larger so it can trip the auto sear on a full auto rifle. It has no function in a semi auto AR15.

Full Auto vs Semi Auto ar15 BCG Comparison

Semi auto and full auto BCGs compared

One benefit of using a full auto BCG is that it will usually weigh about 1 oz. more than a semi auto BCG. This extra weight can sometimes have a positive effect on reliability, depending on the configuration of your rifle. It is the equivalent of going to a heavier buffer i.e. from a carbine buffer to an H buffer, or an H buffer to an H2 buffer.

BCG Finishes and Coatings

BCG finishes photo

BCG finishes L-R: Titanium Nitride, NP3, Nickel Boron, Black Nitride, Chrome, DLC, Phosphate

There are a dizzying array of finish choices for bolt carrier groups today. For the full story, see our in depth report Best AR15 BCG Finishes - Pros Cons and Recommendations.

The short story is this - it doesn't matter as much as you might think.

Standard phosphate finished BCGs have been getting the job done for as long as the AR15 has been around.

In the video below, the Military Arms Channel fires over 7000 rounds through a BCM AR15 without cleaning or added lubrication and without a stoppage. A standard phosphate BCG is used for the test.

If phosphate BCGs can work so well, why choose anything else?

Other options like Nitride, Nickel Boron, and DLC coating can be much easier to clean. They are also smoother finishes that will theoretically function longer with less lubrication, less maintenance, and more corrosion resistance than phosphate.

What is Mil-Spec?

Mil spec gun parts

Mil-spec standards are simply a list of criteria specified by the U.S. military for a particular item. Many factors go into the mil-spec including cost and ease of procurement.

The reality is that mil-spec can be exceeded and it frequently is.

A mil-spec BCG will have:

  • Phosphate finish with chrome lined carrier and gas key
    • There are some tangible benefits to other finishes
  • Carpenter 158 bolt
    • 9310 is a common alternative steel and is up to 7% stronger
  • High Pressure Tested (HPT) bolt
    • Some bolts are individual HPT while others are batch tested
  • Magnetic Particle Tested (MPI) bolt
    • A quality bolt will be MPI. Sometimes it is marked on the bolt, but not always.
  • Properly staked gas key

4 Best AR15 Bolt Carrier Groups

#1 Colt Mil-Spec BCG

If mil-spec is what you seek you cannot do better than Colt. Colt is the gold standard for mil-spec M-16/AR-15 parts.

This BCG has all of the mil-spec details mentioned earlier including the phosphate finish, HPT/MPI bolt and chrome lined carrier and gas key.

Colt Mil-Spec BCG for sale

Colt Mil-Spec BCG

$ 229

Prices Subject To Change

Prices Subject To Change

#2 Aero Precision Black Nitride BCG

A nitride finish is one the most common BCG upgrades and it's an improvement over mil-spec. This super hard and smooth finish cleans up easier than phosphate and slides as smooth as glass from day one.

Aero Precision is a large manufacturer that specializes in high quality AR15 and AR10 parts. Their Black Nitride BCGs are available with or without their logo and it's nice to have that option!

Aero Precision Black Nitride BCG for sale

Aero Precision Black Nitride BCG

$ 149

Prices Subject To Change

Prices Subject To Change

#3 Aero Precision Nickel Boron BCG

Slicker than Black Nitride, Nickel Boron is a state of the art finish that is extremely durable while also being very slick.

Nickel Boron has been proven in the military and is specified on Army 60mm and 81mm mortar tubes, Blackhawk helicopter hoists, and for hard use applications on some Navy ships. See more on the benefits of Nickel Boron here: Top 3 Reasons You Might Want a Nickel Boron BCG

nickel Boron specified on Army Mortar Tubes

Like their Nitride BCGs, Aero offers their Nickel Boron BCGs with or without their logo. The ones with their logo have generally been very hard to get however.

Aero Precision Nickel Boron BCG for sale

Aero Precision Nickel Boron BCG

$ 184

Prices Subject To Change

Prices Subject To Change

#4 Bravo Company Manufacturing BCM BCG

Bravo Company Manufacturing (BCM) has one of the best names in the industry. Known for high quality AR15 rifles, parts and more.

The Bravo Company BCG is as close to mil-spec as you can get outside of Colt. They generally sell for a little bit less but bring all the mil-spec features to the table including phosphate finish and HPT/MPI bolt.

They come with the BCM logo in white. Brownells also offers them in FDE, however they were out of stock as of this writing.

BCM M16 BCG for sale

BCM M16 BCG

$ 189

Prices Subject To Change

Prices Subject To Change

Bolt Carrier Group Frequently Asked Questions

What is a bolt carrier group?

An AR-15 bolt carrier group (BCG) is one of the most important parts of your rifle or pistol. The bolt carrier contains a lot of pieces that work together to facilitate the cycling and ejecting of ammunition within your gun.

What is the difference between a semi-auto vs a full-auto BCG?

The difference between a semi-auto and full-auto bolt carrier group is the shape of the carrier. A full auto carrier will have a larger rear lug so it can trip the auto sear on a full auto rifle. It has no function in a semi-auto AR15.

What is a mil-spec BCG?

A mil-spec BCG will have the following: phosphate finish with chrome lined carrier and gas key, carpenter 158 bolt, High Pressure Tested (HPT) bolt, Magnetic Particle Tested (MPI) bolt, and a properly staked gas key

What type of BCG is the Best?

There are a lot of choices out there, but the short story is, you can't go wrong with the standard phosphate-finished BCGs and if you're looking for something easier to clean with smoother finishes, the Nitride, Nickel Boron, and DLC finishes are also great long-lasting options.

High-Quality BCGs at SpiceTac

For more information on bolt carrier groups, visit our review on The Best AR15 BCG Finishes.

At SpiceTac, we carry Aero Precision, Toolcraft, Failzero, and Spike's Tactical BCGs in a variety of finishes. For the best online shopping experience, check out our wide selection of AR-15 and AR-10 parts. Count on us for your next upgrade with fast shipping and affordable prices on the best parts.

Nov 08, 2021 SpiceTac

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