There are many bolt carrier groups on the market for AR-15s, but not all are made equal. Choosing the best one for your rifle or your custom build will have everything to do with its purpose and operation.
The AR & Its Components
The AR-15 was developed by Armalite and the first batch of rifles was released in December of 1959. The design of the AR-15 is simple, but the rifle’s reliability is totally dependant on component quality.
Fun Fact: The AR in AR-15 stands for Armalite Rifle
The lower part of a rifle is comprised of the serialized lower receiver, stock, buffer system, trigger group, and grip. These components are assembled together with a lower parts kit that includes the controls of the functionality of the firearm.
The upper part of the rifle includes the upper receiver, barrel, gas system, bolt carrier group, charging handle, and muzzle brake. The complete upper receiver has fewer parts to assemble than the lower, but does require a specific torque for the barrel nut to ensure a proper fit. The remaining components include a forward assist button and ejection port cover.
The Bolt Carrier Group
The bolt carrier group (BCG) is what fires the round when the trigger is pulled, ejects spent casings from the chamber, re-cocks the hammer, and chambers a new round from the magazine.
The bolt carrier group is essentially what allows the rifle to fire in semi-automatic mode, and in some rifles, fully automatic. When any component of the BCG is damaged, cracked, or missing, the entire functionality of the firearm can and will fail.
Learning the vernacular of how the BCG operates as well as the proper names of the components is important when speaking about what condition your firearm is in.
These are the components of a bolt carrier group and what they do for a rifle’s functionality:
The bolt carrier is what specifically holds the firing pin, gas key, cam pin, and bolt together. It physically moves backward into the buffer tube as the gun is fired and does the work of chambering the next round when pushed back into battery by the buffer.
The carrier key is also known as the gas key because the gas tube deposits the gas into this key to power the AR’s cycling action. When a bullet travels through and leaves the barrel, the gas from the burning of powder filters through the gas hole in the barrel, backward through the gas tube, and into the carrier key.
This pressure from the gas is what unlocks the bolt, pushes it back against the buffer, and springs back forward to its locked position.
The cam pin keeps everything functioning and holds the bolt and firing pin together. The cam pin was designed to only be able to go into the hole in the bolt one way. This pin prevents the bolt from overrotating when it unlocks.
The firing pin runs through the hole of the cam pin and is locked into place by the firing pin retaining pin. When the firing pin is inside of the bolt and retained, the head of it should be visible, and it should not fall out.
The firing pin is what strikes the primer of the cartridge kicking off the firing sequence of the rifle.
Firing pin retaining pin
The firing pin retaining pin holds the firing pin in place inside of the bolt. Without it, the firing pin would fall out and could cause light or no strikes on the primers, preventing the gun from firing.
The bolt includes the extractor, ejector, and bolt gas rings. The bolt’s face looks like a cog and is what rotates when the BCG locks and unlocks into place inside the firing chamber.
The extractor is part of the bolt and its job is to pull the spent casings out of the chamber of the gun. A smaller component on the face of the bolt is an ejector. The extractor hooks onto the rim of the round pulling it backward and the ejector spring forces the ejector to push against the spent casing as the BCG is moving rearward.
The spent casing is then ejecting out of the upper. As the BCG comes back forward, the bolt head and extractor grab another round from the top of the magazine, chambering it, and locking into place.
Bolt gas rings
The purpose of the gas rings is to trap the gas so the gas moves the bolt instead of gas going into the upper receiver. There are three gas rings that should be checked and replaced as needed. Some AR-15s can still function on two and even one gas ring, but it’s not recommended.
When you hear that the bolt is locked, you probably assume that means the bolt is locked back to the rear, held open by the bolt catch/release. This is incorrect. When the bolt is locked, that means the bolt is locked inside the firing chamber waiting for the trigger to be pulled to begin the unlocking process.
Types of Bolt Carrier Groups
There are two types of bolt carrier groups. The AR-15 bolt carrier is designed to shoot semi-auto while the M16 bolt carrier is designed for full auto. The M16 bolt carrier groups have an extra lug that is compatible with a full-auto sear. These carriers are also a little bit heavier than AR-15 carrier groups.
There are also different categories of BCGs such as mil-spec bolt carriers, half-circle bolt carriers, and low mass carriers, all of which serve a different purpose.
Mil-Spec anything simply means that the design and build of the product are up to US military standards. When it comes to mil-spec bolt carriers, they must be made from 8260 steel, the interior of the bolt carrier must be chrome-plated, is subjected to shot peening, and has Grade 8 fasteners.
These bolt carriers have a portion of the cylindrical end of the carrier milled off to create a half-circle design. The area beneath and behind the firing pin is also milled off to create a lighter bolt carrier.
These lightweight bolt carriers have more of the metal milled off compared to a standard mil-spec carrier group. The low mass BCG reduces felt recoil and the movement of the rifle, so rapid shots cycle faster and smoother. If you change to a low mass bolt carrier group, you may have to adjust your buffer system to match if the BCG is not heavy enough to lock and unlock all the way or have an adjustable gas system to modify the gas to the new BCG.
To add even more complexity, an AR 15 bolt carrier can be made from steel, aluminum, or titanium, and have different coatings.
Steel Bolt Carriers
Steel has been around for a long time. It is durable, heat resistant, and is affordable. The military mil-spec 8620 steel has been used in both full-auto and semi-auto bolt carriers, but not so much in the bolts.
This type of steel has been heat-treated and is made to resist wear and tear. Carpenter No. 158 steel is mil-spec steel used to manufacture bolts. It is more expensive steel, but is more durable for the lifespan of a bolt. C158 steel is case-hardened, meaning the outside of the steel is hard to prevent cracking and the inside is softer preventing stress fractures.
Lastly, 9310 steel is heat-treated as well and is used to make both bolts and carriers. This steel is more readily available and also affordable.
You’ll find that most low mass bolt carrier groups use aluminum since it’s a lighter material. Aluminum is not as durable as steel so you may find yourself replacing your bolt carrier group more often. These also work well in rifles that have an adjustable gas block
Titanium is more expensive than steel but is much stronger than steel BCGs. Titanium weighs less and is heat and pressure-resistant making these BCGs a premium option for AR-15s.
Performance Testing & Coatings
BCGs go through high-pressure testing (HPT) as well as magnetic particle inspection (MPI). High-pressure testing is not something you try at home and this testing can prevent someone from blowing up their hand or face when shooting hot ammunition. Ammunition made by commercial manufacturers has to meet SAAMI-rated specifications.
For HPT, a high-pressure cartridge above the safe SAAMI-rated specs is fired to ensure the BCG is sound and able to handle continuous fire with extreme pressures.
MPI testing is done after HPT. During this process, the BCG is placed inside a magnetic field. A liquid solution containing magnetic particles is applied to the steel. These particles’ job is to stick to any cracks or deformities that are on the surface of the BCG. Ultraviolet light is used to illuminate the liquid and look for any sign of cracks, wear, and any deformities in the steel.
Bolt carrier groups can be parkerized, black nitride treated, nickel boron coated, or coated with titanium nitride.
- Parkerizing: Bolt carrier groups often go through a process of parkerizing to protect the metal from corrosion, scratches, and to make it more wear-resistant. The first step is to clean the surface of the metal to remove all grease, oils, salts, dirt, etc. Next, the metal is immersed in hot water and then submerged into the parkerizing solution. Finally, the metal part is removed and left to hang on a rack to remove any excess.
- Black Nitride: Black nitride is also known as a salt bath. This heat treatment hardens steel which is exactly what you want in a bolt carrier group to resist wear and corrosion. This treatment also makes the surface smooth for easier cycling in a rifle.
- Nickel Boron: Nickel boron offers even higher resistance to wear and corrosion. Bolt carrier groups coated in this finish will have greater lubricity than chrome and nitride BCGs.
- Titanium Nitride: Titanium nitride is used in aerospace and military applications. This is an extremely hard ceramic material, makes cleaning easy, improves edge retention, has good lubricity, prevents corrosion, and its gold color is a nice change from matte black.
The Best BCGs Reviewed
1. Best Low Mass BCG: JP Enterprises
JP Enterprises are known for their extremely reliable modern AR-15s and pistol caliber carbines. The performance of their rifles has been tested by competitors in USPSA, Steel Challenge, 3 gun, PRS, and more.
The action of their low mass bolt carrier group reduces felt recoil and helps the shooter acquire their sights faster for the next shot. One upgrade to make if purchasing this low mass carrier is switching out to a lightweight buffer system.
2. Best Mil-Spec BCG: Daniel Defense
This mil-spec bolt carrier group meets all the requirements of the US Military. The bolt is shot-peened, has an extractor booster for semi-auto or full-auto fire use, and each assembly is magnetic particle inspected.
A huge benefit of the black phosphate model is it has a full-length shroud for tripping auto sear for use in law enforcement and military rifles.
3. Best Titanium BCG: Rubber City Armory
Rubber City Armory makes high quality bolt carrier groups and this M16 BCG is compatible with semi and full auto firearms. It is nitride coated for wear resistance with a bolt machined from 9310 steel and titanium carrier that creates and assembly that weighs just 7.8oz.
Not cheap but when every ounce matters this is the BCG to run.
Bolt carrier groups run the show when it comes to cycling an AR-15. Choosing the wrong one can lead to failure to fire, failure to feed, and failure to eject. If you have a full-auto rifle, there is no other choice than an M16 BCG.
If you’re building a custom semi-auto rifle that you want to be smooth and soft shooting, consider a half-circle BCG. Lastly, if you’re a competition shooter looking to reduce recoil, have faster sight acquisition during recoil, and shoot faster splits, a low mass bolt carrier group will serve you well.
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