What is an M4 Bolt Carrier Group?

If you’re new to the world of firearms, you’re most likely familiar with the AR-15. But surround yourself with experienced pros and they’ll often use the term M4, which can be confusing when they mention things like the M4 bolt carrier group.

To make it worse they will interchange the words AR-15 and M4! What’s the difference? And what are the differences between the BCG’s? 

Keep reading to find out all about what a bolt carrier is and why your friends get so passionate about it. You’ll also find out why the M4 and AR-15 terms are interrelated.

Understanding the Bolt Carrier

The bolt carrier group is the heart of the M4. It performs the repetitive motion that is so important to the purpose of the rifle. The group takes care of all the activities involved with loading and firing the gun. 

Many people refer to this as the “action” of the rifle; without this action, your rifle turns into a useless piece of metal. 

Cleaning

Understanding the cleaning process is important to every gun owner because every shot can cause carbon to build-up. This carbon will eventually hinder the bolt carrier action if left unchecked which is why so many gun owners are fanatics about cleaning. If you don’t clean, you don’t have a functioning weapon. Cleaning, however, can be difficult without the right knowledge and the right tools. 

No matter what type of ace M4 bolt carrier group you get, you want to keep it clean and in working order. 

The M4 and AR-15 Relationship

The original version of the AR-15 came out in the 1950s. The AR stands for Armalite Rifle and not “assault rifle” as so many people think. The government chose the rifle under the name M16. 

The semi-automatic version used by law enforcement and civilians came out in 1964 to great popularity. The AR-15 has stood the test of time and comes in many different variations.

The M4 came as the carbine version of the M16. Its testing was under the name XM4 and the military chose it in 1994. The AR-15 and M4 only differ in their respective exterior appearances.

Plus, the military version of the M4 has a cutout that allows a grenade launcher to be attached. 

AR-15 vs M4 Bolt Carrier Group

The inside of the two rifles is almost exactly the same which is why you will see different listings of the AR-15 bolt carrier groups, but understand that they fit both rifles.

The main difference in the bolt carrier groups is the M4 has the capability of automatic firing of a 3-round burst. The AR-15 only fires as a semi-automatic rifle.

The M4 is specifically made for military use and the AR-15 is for civilian and police use. However, people do have the capability of customizing their AR-15 to give it a similar look and feel to the M4.

M4 Bolt Carrier Group Details

The bolt carrier group M4 works by having different separate pieces that work together to carry out their job. Each of these functions fits a different role and without any one piece, the bolt loses its capabilities.

What It Does

The amazing part of what the bolt carrier group does is how it completes its functions inside a chamber with intense heat and pressure. Here is a quick breakdown of the bolt carrier group’s role: 

• It takes the new round and loads it into the chamber
• It fires the round out
• It’s gas-operated tube impinges the bolt carrier
• It ejects the empty casing out of the gun
• It sets up for the new round by cocking the hammer back
• It repeats this process with every shot

As you can imagine, this process happens every time you pull the trigger which means thousands upon thousands of functions. 

Why It’s Important

A reliable bolt carrier group is important because soldiers rely on it to work every time they need it. Even someone on the range wants their AR-15 to work every time they pull the trigger.

When someone pulls the trigger, the hammer hits the firing pin. The pin will hit the primer on the round in the chamber sending it out with an explosion. The bolt recocks the hammer after expelling the casing and is fed the next round.

This action takes place in milliseconds and if any piece doesn’t do its job, the whole mechanism stops. You now know why so many gun owners are fanatics about cleaning.

Cleaning the bolt carrier group takes time and discipline, but if it doesn’t happen, the bolt can’t function properly. 

M4 Bolt Parts Broken Down

The parts of the M4 bolt carrier group work together but you can also take them apart for cleaning. If a part breaks or wears out, you can replace it without having to replace the entire carrier group.

Here are the main parts of the carrier group:

• External Housing – This part can go by the names “carrier” or “bolt carrier” and is the frame that holds all the pieces together
• Bolt Carrier Key – This part channels the gas into the bolt carrier
• Assembly – Connects to the carrier end and contains the extractor, ejector, and gas rings
• Bolt Cam Pin – A large pin that is short in length and functions to guide the bolt’s movement forward and back
• Firing Pin – The piece that hits the primer in the cartridge and starts the whole process
• Retaining Pin – This small pin keeps the firing pin set up and in place

Each part is important and the great part is each piece can be replaced should anything happen. You want to know how to disassemble your rifle and put it back together to help you with cleaning and maintenance.

The rifle’s design gives its owner options for easy maintenance which is why so many gun owners keep spare parts handy.

They can break down their bolt carrier group, find the piece that isn’t working and replace it quickly. This may not be a big deal on the range, but in battle, the difference could save your life.  

Types of Bolt Carrier Groups

The M4 bolt carrier groups do have some different options available. Gun owners choose them because of their individual preferences and needs.

Full Auto Bolt Carrier Group

The full-auto bolt carrier group is part of the military’s M16 and M4 rifles. They are a little thicker on the back of the carrier. This thickness gives them the ability to trip the auto sear which gives the rifle automatic shooting capabilities.

The extra weight makes this bolt more reliable and smoother in its operation which is why many gun owners prefer to use this bolt carrier. What’s more, the carrier group is not illegal to install in an AR-15.

AR-15s will only shoot like an automatic rifle if the owner has installed an auto sear which are either highly regulated or outright illegal for most gun owners. 

Semi-Automatic Bolt Carrier Groups

These bolt carriers were a version that carried less weight in the back because they were smaller which keeps them from tripping an auto sear. This function keeps anyone from turning the AR-15 into an automatic weapon.

The problem is these carrier groups were not as reliable as the full auto bolts which is why very few manufacturers make them anymore. Gun owners demand reliability out of their rifles. 

Low-Mass Bolt Carrier Groups

These groups, also known as half-circle bolt carriers, were born out of the desire of gun owners for lighter pieces. The guns are still fully-functional, but only have the bare minimum metal needed to make them lighter. 

Some gun owners prefer the benefit of lighter hardware because there is less recoil in the gun which helps shooters to stay on target. The main use for these bolt groups is for competition shooters.

The shooter’s goal is to have a gun with the lightest weight possible for the competition so long-term reliability is not as big of an issue. 

M4 Bolt Carrier Metals

Due to the incredible heat and pressure inside the rifle, the material the bolts are made from is very important. Functionality, reliability, and weight are the main factors that help people to pick different metal types.

Steel

The most common metal for the bolt carrier groups in the M4 and AR-15 is steel. Steel seems to have the right balance of durability and price for most gun owners.

The most common steel types used by manufacturers are a mixture of three common types:

• 8620 steel
• 9310 steel
• Carpenter 158 steel

The firing pin of some manufacturers use 8740 steel and the gas key for others use 4140 steel. All these numbers are used to define the hardness and composition of the steel. 

Titanium

Titanium is a very strong and lightweight metal. Its durability can handle the heat and pressure of the rifle with ease. Many gun owners like the lightweight aspect because it reduces the overall weight of the gun. 

The lightness of the metal also helps to reduce the strength of the recoil. The negative side comes with the cost. Titanium is very expensive and not as easy to work with as steel for manufacturers.

This makes guns of this type less common for gun owners and more for owners who have the funds for a specialty piece.

Aluminum

Many gun owners, especially competition shooters, have switched to aluminum for their bolt metal. Aluminum gives a lighter weight than steel but is not as expensive as titanium. 

The negative side of aluminum is its durability. Aluminum is softer than steel and does wear out faster which can get expensive for gun owners who do a lot of shooting. 

Best M4 Bolt Carrier Group Finishes

When it comes to M4 bolts, many gun owners will argue and battle over the finishing of the bolt more than the materials. There are several different types of bolt carriers and each one has its group of fans.

Some bolts will even have a different mix of these coatings for different pieces.

Parkerized Manganese Phosphate

This is the most common coating and is used for all military applications. It comes with a matte black finish that has texture and feels rough to the touch. Its porous nature helps it to retain lubricant well. 

Salt Bath Nitride

This coating is often called “nitride coating” or “black nitride coating.” The process creates a strong and slick coating around the metal. It is very hard and very resistant to corrosion. 

Its slick look and low friction capabilities make it a very popular coating for the bolts.

Nickel Boron Coating

Nickel Boron is an alloy that gives a metallic coating to the metal. Its friction is reduced so much that it doesn’t require very much lubricant. It also receives high praise from gun owners for being easy to clean.

It is a little more expensive than the other coverings.

Diamond-Like Coating 

Diamond-like coating (DLC) represents carbon that is diamond-like and gives great wear resistance and decreases friction. This coating is one of the best at handling the heat so it resists thermal stress over time. 

There are literally dozens of ways BCG’s can be made, so there is a lot to choose from.

Your M4 Bolt Carrier Group

Depending on the type of shooting you engage in, you know enough about the M4 bolt carrier group to make an educated choice. Since it is the heart of the weapon, your choice does matter.

There are a wide array of BCG’s to choose from. Upgrading your gear can lead to improved performance, which experienced operators will surely notice a difference from their standard issue bolt carrier group. Go forth, enjoy shooting, and make sure to keep your rifle clean with the best tools.

Justin Trump

Justin Trump is the managing editor and owner of CAT Outdoors. The son of a Vietnam veteran, he’s an avid gun enthusiast and 2A advocate. He holds two firearm patents for the CAT M4 and Talon tools. When not managing CAT Outdoors, he enjoys spending time with his family and friends, rooting for Michigan sports teams, and serving his church.