Regardless of whether you own an AK-47 or an AR-15, the bolt carrier group is the heart of the action. When you buy or build an AR-15 chambered in 7.62x39mm Soviet, you need to choose a bolt carrier group that performs to the standard you need.
The bolt carrier group should, ideally, possess a protective coating to resist corrosion. It should be sufficiently strong to withstand high pressures and thousands of rounds. Ideally, the extractor and ejector should be tough enough to cycle various types of ammunition.
Best 7.62×39 AR-15 Bolt Carrier Groups (BCGs)
1. Faxon Firearms 7.62×39 Bolt Carrier Group — Nitride
Faxon Firearms’ 7.62x39mm Bolt Carrier Group is built to last. The bolt and bolt carrier are composed of 9310 and 8620, respectively, which is standard. However, the extractor is made from S7 tool steel. The “S” stands for shock resisting, and S7 provides the necessary strength and hardness to extract stubborn cartridges.
A significant quantity of 7.62x39mm ammunition is steel-cased. Steel, having a lower coefficient of thermal expansion than brass, doesn’t expand and contract to the same extent. This lower coefficient can sometimes cause extraction to become problematic. Furthermore, as steel is harder, this can damage or accelerate wear on extractors. S7, tough tool steel used in the manufacture of mandrills, can withstand high pressures and failures to extract.
In addition to its rust preventative properties, the salt-bath nitriding process also increases wear resistance and lubricity. Not only does this improve the smoothness of the action’s cycle, but it also makes it harder for fouling to adhere to the bolt carrier group, making cleaning easier.
This is a mil-spec bolt carrier group, despite the caliber not being used by the United States: Faxon applies the same processes to ensure reliable operation and durability. These include shot peening to relieve stress and magnetic particle inspection to reveal cracks before they propagate. For increased reliability, the company also chambers the locking lugs at a 45° angle.
2. FailZero EXO Coated 7.62x39mm Bolt Carrier Group
This drop-in bolt carrier group is designed to be compatible with mil-spec upper receivers. FailZero uses a Carpenter 158 bolt and an 8620 alloy-steel bolt carrier. The most novel feature of the FailZero Bolt Carrier Group, however, is the EXO nickel-boron coating.
EXO nickel boron is a proprietary coating technology created by UTC Coatings, the parent company of FailSafe. The EXO coating has a higher boron content than conventional nickel boron (NiB), increasing surface hardness, lubricity, and corrosion resistance to a degree greater than competitors. The company advertises this technology as providing a permanent dry lubricity, which reduces friction regardless of whether you use lubricating oils.
AR-15-pattern firearms that use the Stoner gas system, in which gas is fed directly into the bolt carrier group, are more susceptible to fouling when fired heavily. As a result, they require more thorough cleaning after extended firing sessions. NiB coatings, and especially the one offered by FailSafe, simplify the cleaning process.
3. Anderson Manufacturing Complete 7.62x39mm M16 BCG
Anderson Manufacturing, based in Kentucky, has taken the AR-15 market by storm, offering low-cost, high-quality complete rifles, lower receivers, and other parts. Their stock isn’t limited to the 5.56mm cartridge. The company also produces bolt carrier groups intended for the AK-47 cartridge.
Anderson’s parts are all mil-spec — 100%. The bolt is made from 9310, the bolt carrier is 8620 Carpenter steel, and the gas key is properly staked to ensure long service life. To ensure a high degree of corrosion resistance, the company chrome lines the inside diameter of the bolt carrier. This protects against rust and reduces friction between the bolt and bolt carrier, increasing the smoothness of the operating cycle.
AR-15 vs. AK-47 Action
In a locked-breech firearm, the bolt is the circular or rectangular part that loads and unloads the chamber, locks into the barrel extension, and supports the cartridge case head. By locking the breach, the bolt maintains pressure in the bore to drive the bullet.
In the traditional AR-15 design, The bolt carrier and buffer/action spring assembly allow the bolt to reciprocate inside the receiver to complete the cycle of operation. When the bolt, which has seven radial lugs, enters the barrel extension, the cam track surfaces in the bolt carrier act on the cam pin in the bolt, causing the bolt to rotate and lock.
In the Kalashnikov design, the rotary bolt consists of two locking lugs that engage corresponding recesses in the trunnion.
One of the principal differences between the two systems is how the respective gas systems work. In the AR-15, the Stoner gas system, often described as direct impingement, feeds gas from the barrel through a gas tube extending from the gas block to the receiver. The tube feeds high-pressure propellant gases into a chamber formed between the bolt and bolt carrier. These gases force the bolt carrier rearward, camming the bolt open and retracting it.
The Kalashnikov system, on the other hand, uses a long-stroke gas-piston system. The piston, or operating rod, is attached to the bolt carrier. When gas from the burning propellant is fed through the gas port into the gas cylinder, it applies pressure against the operating rod, forcing it rearward. This rearward movement causes the bolt carrier to rotate and unlock the bolt through a cam track and pin. As the bolt carrier moves rearward, the bolt extracts the spent cartridge case from the chamber until it hits a fixed ejector, expelling the spent cartridge from the weapon through the ejection port.
Ejector and Extractor
The AK uses a fixed ejector attached to the inside of the receiver, which runs in a slot in the bolt carrier. When the cartridge case head strikes the ejector, it pivots about the extractor and flies out of the weapon.
The AR-15 uses a plunger-type ejector that the cartridge case forces into the face of the bolt when seated in the chamber. As the cartridge is extracted from the chamber, the ejector is allowed to move forward, gradually tilting the casing to the right. When the front of the cartridge clears the chamber, the spent round is ejected.
Pressure Differences
The 5.56mm cartridge operates significantly higher than the 7.62x39mm — 55–62,000 psi vs. approx. 45,000. As a result, the bolt must withstand these increased pressures to ensure reliable, safe operation.
Many AR-15 manufacturers choose steel alloys such as 9310 and Carpenter 158 steel for the bolt, which are strong and tough. AISI 8620 is standard for the bolt carrier. These alloys are widely used in the manufacturing industries.
You’ll also often see companies manufacturing parts and assemblies for rifles advertise shot peening, high-pressure testing, and magnetic particle inspection.
● Shot peening
Shot peening relieves stress from steel by bombarding it with shot to induce plastic deformation and change the mechanical properties of the metal part. This process can extend the service life of the bolt.
● High-pressure testing
High-pressure testing is also called proof testing and involves firing a proof load using the company’s bolt carrier group. A proof load generates chamber pressures exceeding the standard for that cartridge. Exceeding the standard is intended to reveal any hidden flaws in the material, ensuring the shooter’s safety.
● Magnetic particle inspection
MPI is used to detect surface cracks and other flaws that may not be visible. The manufacturer passes the bolt carrier group through a magnetic field and applies iron powder or other ferrous particles to the surface of the bolt carrier group. These particles are suspended in water or oil and cover the entirety of metal they are testing. The ferrous particles cause magnetic flux leakage and reveal cracks under UV light that are normally hidden to the human eye.
Final Thoughts
The 5.56mm cartridge is not the only caliber option available for AR-15-platform weapons. For those who want more punch from their AR-15 build, consider the 7.62x39mm Soviet cartridge. This round delivers more kinetic energy and mass to the target, albeit at a lower velocity and more recoil. Take advantage of this classic Soviet caliber in an AR-15 for the ultimate combination.
The bolt carrier group is one of the most critical assemblies of your rifle, after the barrel. While most manufacturers focus on delivering a product that adheres to military specifications, some go a step further.
