Bolt Carrier Groups

Bolt Carrier Groups

By Firefield  

If you are in the market for your first—a or new—AR-15, you are probably overwhelmed by the huge variety of choices available. From budget to high-end to build your own, it probably feels like there are just too many options. Your head is swimming with all the decisions you need to make—direct gas impingement or gas piston, pistol, mid, carbine or rifle-length gas system, handguards, furniture, forward assist, barrel length and type and on and on and on… Some of the parts on an AR are more important than others, and one imperative AR part you need to put much consideration into is the bolt carrier group or BCG.

The bolt carrier group is the heart of your AR-15.

What is a Bolt Carrier Group?

The bolt carrier group is often referred to as “the heart” of your AR-15. It consists of the bolt, bolt carrier, firing pin, carrier key, bolt gas rings, cam pin, firing pin retaining pin and the extractor. These parts are essential to the operation of your rifle. Without the BCG, your gun couldn’t fire a bullet. It performs the firing cycle of loading a round into the chamber, extracting the spent cartridge and then loading the next round and firing it until your magazine is empty, as well as resetting the hammer.

  • The bolt is what grabs a round from the rifle’s mag and then ejects it once the bullet is fired.
  • The firing pin is what hits the primer at the bottom of the round causing the bullet to release from the cartridge and fly down the barrel.
  • The carrier, with the gas key attached at the top, holds the bolt.

Durability is extremely important when picking out a BCG because it takes most of the gas pressure when the rifle is fired. If you skimp or cut costs on your bolt carrier group, your AR-15 has a higher chance of failure and malfunctions.

How to Pick out a BCG

You do have the option to buy all the parts that make up the BCG separately. Particular AR-15 owners, like professional competitors, want to cherry-pick their parts, while other AR shooters choose instant gratification and buy a complete BCG. Neither is right or wrong—which type you chose depends on your needs and circumstances. For example, for casual plinking and target shooting, a standard MIL-SPEC BCG will do the job and do it well.

There are three different types of bolt carrier groups—semi-automatic, full-auto and low-mass.  A semi-automatic BCG is rated for a semi-auto rifle only, is generally made to MIL-SPEC and is often standard. The full-auto BCG is rated for full-auto rifles, is longer and weighs more than the semi-auto BCG. It is markedly different when you look at a full-auto BCG next to a semi-auto one. There is more metal and an extra lug on the back of the full-auto BCG. Because of this, it is heavier than the semi-automatic bolt carrier groups. (Full-auto bolt carrier groups do not make your rifle fire in full auto.) These are becoming increasingly popular for the average AR owner because they slow down the cyclic rate creating a smoother action and greater accuracy. It also shrouds the firing pin and helps keep the case reusable if you reload. However, if you choose to go with a full-auto BCG, you need to make sure you also have a heavier buffer to accommodate.

A bolt carrier group field stripped into its basic components.

A semi-automatic BCG is often MIL-SPEC and you should never purchase anything less. Many people get stuck on the buzzword MIL-SPEC; however, it only means the product meets the minimum criteria for the military’s standards laid out many, many years ago.

Those MIL-SPEC requirements are:

  • Made of Carpenter 158 steel
  • Chrome-plated interior
  • Incorporate Grade A fasteners
  • Shot-peened
  • Parkerized finish

Advances in technology since have made these standards seem archaic now.

Speaking of advances, the low-mass BCG is the latest to hit the market and has become the benchmark for 3-Gun competitors. They are extremely lightweight reducing felt recoil, increasing the rate of fire and allows shooters to be quicker and more accurate on follow-up shots. However, this comes at a price. Low-mass bolt carrier groups wear out quicker and users will need to make sure their rifle has an adjustable gas system.

What are BCGs Made of?

A full-auto rated BCG has more metal in the rear.

Bolt carrier groups are generally made of steel, but there are also titanium and hybrid BCGs.

Steel BCGs are the least expensive and most widely available. The best steels to look for are 9310, 8620 and Carpenter 158 steel. Steel is used because it is strong, resists heat and withstands abuse. A bolt carrier made of one solid piece of steel is ideal because you don’t have to worry about the gas key being properly staked.

The gas key sits on top of the bolt carrier and connects to the gas tube. In a gas impingement system, it gathers the high-pressure gas and pushes it into the bolt carrier. Staking is a way to make sure the screws on the gas key never come loose. If they come loose, gas will escape and your AR-15 will malfunction.

Weighing half as less than steel, but much stronger is titanium. Titanium is highly resistant to heat but is more expensive than steel.

Hybrid bolt carrier groups will be constructed of different metals to eliminate weight where possible while reinforcing strength where needed.

Bolt Carrier Group Coatings and Their Advantages

Phosphate/Manganese Phosphate

This is the most common coating for bolt carrier groups because it is affordable and durable. The BCG is submitted to a bath of phosphoric acid heated to 200 degrees A phosphate-coated BCG is heat- and corrosion-resistant but needs lubrication to run smoothly and is more difficult to clean than other coatings.

Nickel/Nickel Boron/Nickel Boron Nitride/NP3

A nickel coating, or any of its variations, puts a layer of nickel plating on the bolt carrier group. It helps the AR run reliably without lubrication, but it isn’t as durable as phosphate or titanium nitride and it’s also an expensive process.

Titanium Nitride

Converting titanium to a gas and creating a reaction with nitrogen deposits a very thin film over the bolt carrier group resulting in a highly corrosion-resistant finish. You get a rifle that will function without much lube and is easy to clean. However, titanium nitride is an expensive finish, and some might not appreciate the gold-toned flashy finish.


This durable coating is produced by dunking the BCG into liquid Meloniting salts heated to 1,100 degrees. It is easier on the steel than other coatings and creates a Rockwell hardness of 68-72. A Melonite-coated BCG is easy to clean, helps reduce fouling and will last the longest.

Finishes and Extras

A properly staked gas key ensures a smooth-running AR.

Magnetic-particle inspected (MPI) This process is a way to check for cracks and inconsistencies in the metal that are so minuscule they can’t be seen by the naked eye. It is done using an electromagnetic solution and UV lights.


Shot-peening is a finish that adds an extra layer of durability to the BCG. Very small pellets are rapidly shot at the BCG.

High-Pressed Tested

There is some deliberation if high-pressured testing is necessary. High-pressure testing is performed on the finished bolt carrier group to make sure it can withstand higher-pressure 5.56mm loads. Companies will either test the finished BCG in batches or individually. This requires actually firing the BCG in a test rifle before the BCG is sent out. Some argue that a high-pressure test decreases the life of your bolt carrier group. This is something you will need to research and decide for yourself.

Since the BCG is such an essential part of your rifle, spend the time researching and reading reviews about the best one to suit your needs. The time, effort and cost pay off in the end to have a reliable, accurate and smooth-running AR-15.

Have you had BCG success or failures? Tell us about your experience in the comment section.

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