The AR15 in 223 Remington, 5.56 NATO and 300 Blackout (and the AR10 in 308) has been called the “Barbie” of rifles and with good reason. Because of the rifle’s modular design there is a dizzying array of parts and accessories for it. In fact, it rivals and maybe even surpasses the Colt 1911 pistol in this regard. Through proper selection of stocks, grips, barrels and other parts it is possible to configure an AR15 for any task from CQB to plinking to home defense to long range shooting. The recent explosion of interest in the AR pistol expands even further the versatility of the design.
The bolt carrier group, arguably the most critical of all AR15 parts to the proper function of the AR, is available from different manufacturers with different specs, dimensions, colors, materials and finishes. It can be confusing and almost overwhelming to navigate through all the specifications in search of the right BCG to get. Fret not, read on.
Seen in the diagram above with a disassembled Toolcraft Nickel Boron BCG are the main components of a bolt carrier group.
These are the parts you will have when you field strip an AR15 BCG for cleaning. The extractor does not need to be removed every time unless needed for cleaning, inspection or to replace the extractor spring - which should be done at 5000 round intervals. The bolt and the bolt carrier w/ gas key can be further disassembled but it is not recommended.
Phosphate is a popular finish and is mil spec for M16 and M4 bolt carrier groups. This finish protects the metal and has a porosity that allows it to “hold oil”.
Black Nitride (also called Salt Bath Nitride, Melonite etc.) is an extremely hard finish with a smooth surface that offers excellent wear and corrosion protection. It’s not a plating but rather a treatment that hardens the surface of the metal. It can have a very nice matte finish like the Black Nitride Toolcraft BCG at the top of the page.
Nickel Boron (NiB) is an advanced plating used on heavy use machine parts where extreme corrosion and wear protection are required. It has a very low coefficient of friction meaning it is naturally slick even when completely dry. A Nickel Boron bolt carrier group can go longer between cleanings due to this slickness and cleans up easier and faster when it does need to be cleaned. One popular brand of nickel boron coated BCGs is the Fail Zero BCG.
See this related blog post Nickel-Boron-BCG-Top-3-Reasons-Your-AR15-Needs-One.
Two of the more common tests performed on AR15 bolts during manufacturing are High Pressure Testing (HPT) and Magnetic Particle Inspection (MPI). HPT involves the firing of a high pressure test cartridge. MPI involves the analysis of the grain structure of the steel with special equipment and it ensures the steel does not have any micro cracks.
Depending on the manufacturer, AR bolts will receive either HPT or MPI testing or both. Military specification (mil spec) requires both HPT and MPI. The argument for doing MPI only is that HPT is semi-destructive testing that unnecessarily stresses the bolt and weakens it. The most important consideration when choosing a bolt is to purchase from a reputable manufacturer that stands behind their product.
AR-15 bolts are available in several different types of steel with the two most common being Carpenter 158 and 9310 steel. Carpenter 158 steel is the mil spec for military bolts. 9310 is a more modern steel that can be approximately 8% tougher than Carpenter 158. They are both premium steels for AR15 bolts and both are widely used and very popular. Either will serve you well!
The original design of the AR-15 bolt did not leave a lot of space for an extractor spring and the standard spring was, I’ll say it, small and weak. At the relatively low chamber pressures and slow bolt unlock times of the original 20” AR-15 this was not an issue. However, as barrels and gas systems have gotten shorter, greater stress was placed on the extractor. It was in these shorter carbines that problems started to occur. Failures to extract primarily.
The U.S. Military’s SOPMOD M4 upgrade program addressed this issue through the use of heavier buffers (to slow the bolt unlocking), and a Viton O-ring that was inserted over the original standard extractor spring. This worked well and largely solved the problem.
Another fix is to use extra power extractor springs. When these extra power springs are used the Viton O-ring is usually not needed. If both are used together it can increase the extractor tension to the point where failures to feed can occur. As a general rule, use either an O-ring with a standard spring or use an extra power spring without the O-ring. Most of us will be fine either way. But, your configuration will determine what is best and a 7.5” suppressed full-auto AR pistol may well need all the extractor tension that it can get. Testing will always tell the tale.
There are two basic carrier profiles, M16 (full auto) and semi auto.
The difference between the two can be seen in the photo above. The M16 carrier has more metal near the rear. This dimensional difference on the full auto carrier is necessary to trip the auto sear on a fully automatic M16 or M4. Both types are generally legal for use in AR-15s. An M16 carrier weighs approximately 0.5 ounces more than a semi auto carrier.
This is the equivalent of about half the weight you would get by going to the next heavier buffer (H, H2, H3). For those shooters trying to add weight to the reciprocating mass of their BCG, a full auto BCG can help to the tune of about a half-ounce.
The most important consideration when purchasing a bolt carrier group is getting one from a quality manufacturer. Toolcraft, Fail Zero, BCM, Colt and Daniel Defense are some of the manufacturers with a reputation for quality bolt carrier groups. Everything else comes down to personal preference.
If a phosphate finished BCG with Carpenter 158 steel bolt rides in your upper receiver, rest assured you are in good company and your BCG will soldier on, serving you well for many years. If a Nickel Boron BCG with a 9310 steel bolt is on the menu for your new AR pistol build, you’ll appreciate the modern technology that went into its construction every time you feel its buttery slick finish. If your BCG has a semi-auto carrier you would probably never know unless you read this article and it will work just as well as a full auto carrier, just 0.5 ounces lighter.
When it comes to parts and accessories for the All-American black rifle, there are many great options available today and your Granddad could not have imagined half of them back in the 1960s. Have fun choosing your favorite parts for America’s favorite rifle.
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