Top 5 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. If you want dependable, bet-your-life reliability from your rifle, a high quality BCG should be a top priority.

With so many BCGs to choose from, it can quickly get confusing. 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 is an upgraded finish on this 5.56 BCG

Contents

Semi Auto vs Full Auto BCG

BCG Finishes and Coatings

What is Mil-Spec?

Recommendations - 5 Best AR15 BCGs

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 group 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, fire and eject rounds as you fire. 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, the hammer rotates forward, hitting the firing pin. The firing pin will then strike the primer of the chambered round and the gun will fire.
• 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 rearward, compressing the buffer and buffer spring. To handle the high temperatures and pressure levels, you'll find bolt carriers and bolts are made from very high quality ordinance steel.
• Bolt - The bolt assembly features locking lugs that rotate to lock and unlock the bolt inside the chamber. As the bolt carrier drives backward, the cam pin rotates the bolt, unlocking it from the chamber and allowing the complete BCG to move rearward.
• 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 - The extractor hooks onto the rim of your round, pulling it out of the chamber.

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.

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.

See our complete article on this topic here: Full Auto vs Semi Auto BCG

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.

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

The incredibly slick Aero Precision Nickel Boron BCG is our #1 pick for an AR15 BCG. 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

Aero offers their Nickel Boron BCGs with or without their logo. As far as logos go, the Aero logos on their BCGs are very well done and look great. They also offer non-logo versions as well.

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.

With this Black Nitride Aero BCG you are getting all the Aero quality and most of the Nickel Boron slickness while saving a few dollars.

This might actually be the best bang for the buck in the BCG world right now.

Again, logo or no-logo, your choice

Diamond Like Carbon (DLC) is a favorite coating in heavy industry for use on drill bits and other extreme hard use equipment. It's a tough, corrosion proof coating that has a low coefficient of friction and bonds so tightly to the base metal that it's nearly indestructible.

When applied to an AR15 BCG it really takes it to the next level. Available in a few different colors including Black, FDE and Gray, this finish is one of our favorites and deserves a hard look if you demand the best for your AR-15.

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.

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.

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.

For information on bolt carrier group finishes, 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.