The bolt carrier group is a vital assembly in the AR-15 platform action that enables your rifle to complete the cycle of operation. While the standard mil-spec bolt carrier group is sufficient for most purposes, the market offers a wide variety of alternatives, many for specialized purposes.
Best AR-15 Bolt Carrier Groups (BCGs)
1. Bravo Company Manufacturing Bolt Carrier Group — M16
Bravo Company Manufacturing (BCM) has a reputation for producing high-quality and reliable guns and parts. BCM’s bolt carrier groups are no exception. Built to last, if you need a mil-spec bolt carrier group, this is the standard.
The inside diameter of the bolt carrier and the gas key are chrome lined, which increases lubricity and corrosion resistance. The Carpenter 158 bolt is shot peened, per military requirement, to relieve internal stresses caused by machining and heat treatment.
High pressure testing, also known as proof testing, ensures that the bolt has no potentially catastrophic flaws that may damage the weapon or injure the shooter. After firing a high-pressure round, BCM conducts a magnetic particle inspection to reveal any cracks.
2. Geissele Automatics 5.56 Enhanced Bolt Carrier Group — Nanoweapon
Geissele Automatics, known for manufacturing competition-grade trigger assemblies in use by special operations forces, has taken a novel approach to bolt carrier groups. On the surface, the Enhanced Bolt Carrier Group appears to be mil-spec. The bolt is Carpenter 158, the bolt carrier is 8620, and the gas key is staked and torqued. However, that’s where the similarities end.
While the bolt is manufactured from Carpenter 158, Geissele collaborated with Carpenter Steel to produce an alloy variant with fewer impurities called Carpenter 158+. One of the most notable features of the Enhanced Bolt Carrier Group, however, is that it’s forged. The forging process compresses the grain structure of the steel, increasing its strength and service life compared with standard billet-machined parts.
The cam pin is under considerable stress and is a known failure point. Geissele chose to manufacture this part using H13 tool steel, which is hard and tough, reducing its susceptibility to breakage.
The company then applies a chrome-nitride coating called Nanoweapon, which increases both wear and corrosion resistance. As Nanoweapon is applied at low temperatures, there’s no risk of the treatment affecting the mechanical properties of the steel.
3. ODIN Works Low Mass 223 Nitride Bolt Carrier Group
If you need to reduce the time interval between shots, there are various ways to accomplish this goal. One is to upgrade your fire control group, reducing the weight, pre-travel, and reset. A consistent release and a short reset can increase your effective firing rate.
Another is to alter the weight of the bolt carrier group, which can increase the bolt velocity and cyclic rate of your rifle. ODIN Works’ Low Mass 223 Nitride Bolt Carrier uses a 9310 alloy-steel bolt and 8620 carrier. However, a key difference from standard bolt carrier groups is that ODIN Works has machined several slots into the carrier to reduce weight. The result is a 9-oz. bolt carrier, which represents a 2.5-oz. weight reduction compared to standard mil-spec offerings.
While there are lighter options available, which are machined from aluminum, these do not offer the same longevity.
You may need to adjust the gas system or buffer and action spring assembly to ensure reliable performance, however. Bear in mind that a low-mass bolt carrier group, in reciprocating at a higher velocity, will increase wear.
For added corrosion resistance, ODIN Works applies a salt-bath nitriding treatment, which also provides a hard wearing surface and increasing lubricity.
4. 2A Armament Titanium Regulated Bolt Carrier
Although a lightweight steel bolt carrier can improve your rifle’s performance, it’s not the lightest option available. Unfortunately, many ultra-lightweight options, such as those made from aluminum, are less durable and experience a reduction in service life. The solution, then, is to substitute a material that is lighter than steel but stronger than aluminum. 2A Armament designed a composite bolt carrier group consisting of a Grade 5 titanium (6AL-4V) main body and a 7075-T6 aluminum rear. This reduces the weight of the bolt carrier group to a mere 5.95 oz. — almost half the weight of a standard mil-spec BCG.
However, a reduction in the weight of the bolt carrier group can require an adjustable gas system. In an AR-15 that uses the Stoner gas system, the bolt carrier group acts as the gas piston. Rather than requiring the installation of an adjustable gas block, the company designed an adjustable bolt carrier group.
The Titanium Regulated Bolt Carrier, or RBC, features a gas regulating gate that you can adjust using an Allen wrench through the ejection port. The RBC is highly adjustable, allowing you to set the gas valve from fully open to fully closed.
2A Armament supplies you with a special-purpose Allen wrench to adjust the Allen-head screw in the right side of the bolt carrier.
In addition to these novel features, 2A Armament also delivers the quality control that you expect from a modern manufacturer of small arms.
While the bolt carrier is a composite, the bolt is standard 9310 steel, cryogenically stress relieved and treated with a QPQ nitride finish. The aluminum rear is hard-coat anodized for increased wear resistance.
5. JP Enterprises Vmos AR-15 Variable Mass Bolt Carrier
JP Enterprises has a reputation for building smooth, high-quality rifles for competitive shooting sports. The company also manufactures a variety of replacement parts and assemblies to enhance AR-15-platform rifles generally.
Sometimes optimal adjustment and tuning require changing the weight of your bolt carrier group. While there are lightweight/low-mass and standard-weight bolt carrier groups and various options in between, JP Enterprises also offers a variable-mass bolt carrier.
Using a series of exchangeable weights, the Variable Mass Operating System (VMOS) carrier allows you to adjust the weight of the bolt carrier group to fine-tune the cycle when an adjustable gas block is not available. If you’re experimenting with sound suppressors or unusual calibers, this can take some of the guesswork out of finding the perfect balance to ensure reliable functioning.
As is customary for the firm, the inside diameter of the bolt carrier is roller-burnished to ensure the smoothest possible operation, and JP applies a QPQ nitriding treatment for increased surface hardness and corrosion resistance.
For the best combination, you can use JP Enterprises’ Silent Captured Spring, which virtually eliminates the “twang” and vibration associated with traditional AR-15 action springs and buffer assemblies.
6. Spikes Tactical M16 Bolt Carrier Group — Lightweight Nickel Boron
Unlike previous lightweight entries, the Spike’s Tactical M16 Lightweight Bolt Carrier Group reduces the weight by one ounce. While that may not seem like much, every ounce counts, and not every AR-15 rifle requires a significant reduction or increase in weight for tuning the action.
Spike’s Tactical achieves this weight reduction by drilling 12 holes through the carrier walls and four blind holes on the left side, opposite the ejection port.
Aside from the weight-reducing holes, the bolt carrier is full-auto mil-spec, made from 8620 alloy steel and featuring a staked gas key. The bolt is composed of Carpenter 158 steel, shot peened, high pressure tested, and magnetic particle inspected, ensuring that it will function flawlessly out of the box.
Last but not least, Spike Tactical applies an electroless nickel-boron (NiB) coating to the entire bolt carrier group — bolt, carrier, and gas key. Nickel boron protects steel parts against corrosion and wear while reducing friction. NiB has become an increasingly popular alternative to hard chrome in recent years.
7. Spike’s Tactical Complete 9mm Bolt Carrier Group
Not every AR-15-pattern weapon is chambered in an intermediate-powered cartridge or operated by gas. Several companies have begun manufacturing AR-15-pattern firearms — both carbines and pistols — chambered in rimless handgun cartridges, such as 9mm Luger, .40 S&W, and .45 ACP. The pistol-caliber carbine provides the advantage of ammunition, and sometimes magazine, a commonality between your sidearm and long gun. It also provides you with a more stable firing platform.
A 9mm AR-15 uses the simple blowback method of operation. In this system, there is no positive mechanical locking between the bolt and the barrel extension. Instead, the inertia of the bolt, the tension of the action spring, and the adhesion between the cartridge case and the chamber resist the opening of the breach until the pressure in the chamber declines to a safe level.
If you’re building a pistol-caliber AR-15 in 9mm, you’ll need a 9mm bolt carrier group and Spike’s Tactical delivers. It machines its 15-oz. matte-black 9mm blowback bolt carrier group from 416 stainless steel.
8. Lantac USA M-SPEC 5.56 Bolt Carrier Group — Titanium Nitride
The Lantac M-SPEC Bolt Carrier Group is mil-spec for the most part, featuring an 8620 alloy steel carrier and shot-peened Carpenter 158 bolt. However, of particular note is the CP-R360 domed-head cam pin, which is made from 17-4PH stainless steel. Lantac hardens and polishes each cam pin by hand to reduce friction inside the upper receiver, causing less wear in the process.
However, Lantac doesn’t stop there. It applies a titanium nitride coating to the bolt carrier, increasing corrosion and wear resistance while reducing friction, delivering a smooth-cycling rifle. The titanium nitride coating gives the bolt carrier group a distinctive gold appearance that causes it to stand out.
AR-15 Bolt Carrier Group
The bolt is a crucial component of any rifle. In most firearms, the bolt is circular or rectangular. Its purpose is to seal or obturate the breach — by inertia, mechanical disadvantage, or by positive locking — support the cartridge case head and facilitate loading and unloading.
In the AR-15 design, the bolt has seven radial locking lugs — the extractor occupies the position of the eighth. As the bolt enters the barrel extension, sometimes called the barrel socket, a cam pin rotates the bolt 22.5°, causing the bolt lugs to engage corresponding recesses in the barrel extension. When the bolt is fully locked, you can fire the rifle.
The bolt carrier can be likened to a steel sleeve that encircles the bolt, carrying it forward and rearward inside the receiver. In the Stoner gas system, propellant gases enter the bolt carrier group through a gas tube and via the gas key. From there, the gases enter a chamber formed between the bolt and bolt carrier. The gases, continuing to expand, apply pressure against all surfaces.
In the process, they force the bolt carrier rearward. The rearward stroke causes the bolt carrier to cam the bolt open, unlocking it from the barrel extension and withdrawing it from the chamber.
Semi-Auto vs. Full-Auto Bolt Carrier
There are two types of bolt carrier groups for the AR-15 platform: semi-automatic and fully automatic. Unless you have access to a selective-fire — i.e., NFA — AR-15-pattern firearm, there is little difference between the two types. The full-auto bolt carrier can activate the auto-sear, enabling fully automatic fire functionality. The semi-automatic bolt carrier, however, is incapable of achieving this result.
Why Upgrade?
There are several reasons to upgrade the bolt carrier group. Numerous manufacturers produce bolt carrier groups that are heavier, allowing for more positive and reliable cycling, and lighter bolt carrier groups to increase the bolt velocity and cyclic rate while also reducing felt recoil. Different platings and coatings can increase the reliability of function and minimize wear. There is also rust prevention, as most bolt carrier groups are composed of steel alloys.
The advantages of a bolt carrier group upgrade range from the minor to the novel. Consider experimenting with different assemblies to find the one that best suits your rifle.
Final Thoughts
The bolt carrier group that you choose for your rifle can play a significant role in both its fit and function. This can affect the smoothness of the cycle, the intensity of the recoil impulse, the longevity, and other factors.
