The Bolt Carrier Group (BCG) is easily considered the heart of the AR platform. Rightfully so, considering the BCG is responsible for a large portion of the rifle's function. This article will go over some of our recommendations and things to look for when purchasing a bolt carrier group.
The Aero PRO BCG is a fairly new addition to Aeros lineup. This is a step above their standard option, but it is well worth the extra cost as the BCG is one of the most critical components of your rifle. The PRO BCG features an 8620 carrier, a 9310 bolt, and a 4140 Extractor, Cam Pin, and Gas Key. C158 is our preferred bolt material, but 9310 can be stronger if properly heat treated.
BCM has long been one of our recommendations. They're simple, and they get the job done. The BCM bolt carrier group features a C158 bolt, tool steel extractor/ejector, and a properly staked chrome-lined gas key all housed in a phosphate carrier with a chrome-lined bore.
Much like the BCM, this is another no-frills option that has a few enhancements over your standard "Mil-Spec" bolt carrier group. This BCG features a C158 bolt, 4340 extractors, and an S7 tool steel ejector. The gas key is made out of 4130 steel, is chrome lined as it should be, and includes enhanced Sprinco extractor springs, a sealed gas key, and properly staked gas key screws. The carrier is made out of 8620 steel and has a phosphate finish with a chrome-lined bore.
You'll notice up until now we haven't recommended any special coatings. That's because a lot of the stuff currently being offered is marketing snake oil. A good Parkerized/Phosphate BCG will perform better than a cheap BCG that's marked up for a coating that doesn't offer much over standard offerings and can cause more issues.
NP3 or Nickel PTFE is one option that offers a benefit over standard coatings. NP3 offers improved lubricity, better corrosion resistance, and is easier to clean. Since NP3 is relatively thin, it is not prone to chipping or cracking like some cheaper Nickel Boron options available today. This topic can get exhaustive, so if you want to dive deeper, you can check out the short write-up Sionics has on their website.
Now that we're back on track, Sionics' bolt carrier groups have an NP3 coated C158 bolt with a tool steel extractor and ejector. This BCG also uses an enhanced Sprinco extractor spring and Optimized Carrier Key Screws (OCKS) to ensure the gas key doesn't come loose.
Hard chrome plating was the original coating used in the late 60s on the XM16 Prototypes; Chrome provides superior corrosion resistance and improved lubricity, much like NP3. Chrome BCGs aren't as common these days as they are expensive to produce, but they are an excellent option for those looking for a coating better than the standard phosphate. BRT's BCG uses a Chrome C158 bolt and full chrome gas key with properly staked fasteners.
The LMT Enhanced BCG has an altered cam path that increases dwell time which can help tame suppressed or over-gassed systems. The LMT Enhanced BCG also features three gas ports instead of the standard two-port system, allowing better gas flow, and since the ports are angled forward, it directs excess gas away from the shooter. The carrier also has relief cuts on the bearing surfaces, allowing it to cut through dirt and debris.
The bolt uses a proprietary steel far stronger than C158 and 9310, pair this with the modified bolt lug design, and you have an extremely strong bolt. LMT also uses a "lobster tail" extractor which uses two springs instead of one. This creates stronger, more consistent extraction and acts as a failsafe if one spring wears out or breaks. LMT has also shaved off the lug opposite the extractor, evening the load between the remaining bolt lugs.
SureFire designed the OBC with the help of Jim Sullivan, one of the original designers of the AR/M4 platform. With its shortened gas key, the OBC has 60% more travel past the magazine, increasing feed time and reducing the chance for feed-related malfunctions. Much like the LMT, the OBC increases dwell time, lowering chamber pressures by 15% before unlocking. This helps control bolt velocity when shooting both suppressed and unsuppressed and reduces gas blowback. The OBC also has a counterweight in the tail section that lengthens lock time and has a dead blow effect when the bolt returns home, which helps eliminate bolt bounce.
Building out a Custom Bolt Carrier Group
Manufacturers We Recommend
Microbest - Is well known for strict QC and very efficient gas sealing.
Schmid Tool - Is well known for triggers; they make most of the OEM triggers out there and offer BCGs.
Toolcraft - Is well known for a good generic BCG with many options.
For the gas key, chrome-lined with a phosphate coating is preferred. This should be made out of 4130 or 4140 steel. After this, we recommend honing the sealing surface of the gas key to ensure it is flat.
The standard grade 8 Mil-Spec hex fasteners will work fine, but if the option is present, Michigun OCKS gas key screws will be preferred. Because of the notches built into the screws, these are least likely to come loose under auto fire and heavy use.
As a secondary seal, a sealing compound should be used on the mating surfaces. This is crucial to attaining a complete seal and good gas efficiency. Permatex or Loctite 620 bedding compound is the preferred sealant.
A hard, slick coating like NP3, Chrome, DLC, or Nitride is preferred for the bolt. This will add some lubricity and aid in the locking/unlocking of the bolt. The bolt should be made out of C158, but 9310 is acceptable from reputable manufacturers with high QC standards.
The ejector should have a beveled edge and ideally be made out of S7 tool steel. Sprinco ejector springs are also recommended in this application.
The extractor should be made of 4340 or S7 tool steel and phosphate coated to aid in gripping the case. Polishing the forward lip of the extractor is a bonus but not necessary.
Sprinco 5 coil extractor springs are a worthy upgrade while you are at it. Barrels under 14.5 should use the insert and rubber O-ring that come with the Sprinco kit, while barrels over 14.5 are generally good with just the insert.
For carriers, the standard is usually 8620 steel, a chrome-lined bore is preferred, but this isn't achievable with coatings such as Nitride. If budget isn't a concern, we would recommend an NP3 or chrome coating. If budget is a concern, the standard phosphate coating works well at retaining oil and allows the carrier bore to be chrome lined. We suggest full mass, M16 profile carriers for duty/defense guns.
Firing Pin, Cam Pin, And Gas Rings
For the firing pin, we suggest 8640 steel with chrome plating; this is generally the industry standard, so that won't be too hard to source.
We prefer the cam pin to be made out of 4340 steel with a slick coating such as Chrome, NP3, DLC, or Nitride. This will lengthen the pin's life and help smooth out the action. Some companies offer dimpled cam pins to help the end-user install the cam pin in the same orientation each time they reassemble the BCG. This allows for consistent, even wear and longer service life.
Last but not least, gas rings. Gas rings are a consumable item, but a quality set like Sprinco will generally last longer. And since these are small, inexpensive parts, the upgrade isn't costly.
If you have any further questions, don't hesitate to email us at firstname.lastname@example.org.