Let me clarify the headline. The Bolt Carrier Group or “BCG” is one half of the AR’s engine, with the barrel assembly comprising the other half. Among the different AR-15 accessories, the BCG is considered the workhorse of the AR-15. It is complex and full of moving parts – enduring extreme temperatures and pressures while reliably cycling round after round. It is for these reasons that I consider the Bolt Carrier Group to be the heart of the AR.
Proper selection of a good BCG can mean the difference in dire situations. When it comes to reliability and service life, your bolt carrier group is your most valuable part. The mechanics of your rifle’s action depend on your bolt carrier group. There are a ton of aspects which come into play when choosing a BCG for your rifle. The BCG is inevitably the most important and complex part of an AR-15.
There are a ton of aspects which come into play when choosing a BCG for your rifle. Unfortunately, the BCG is the most often overlooked and/or skimped on component by individuals when they are assembling or “upgrading” their AR. The goal of this article is to tell you what features and characteristics you should look for in an ideal BCG. So let’s get on with it!
What is a Bolt Carrier Group (BCG)?
BCG stands for Bolt Carrier Group. BCG’s are found between the upper and lower receivers of an AR and perform all the basic functions of the rifle. The BCG offers the AR-15 semi-automatic firing capabilities, which means you can fire rounds consecutively without the need for pulling the charging handle after every shot.
Let’s take a simplified look at the functions of the BCG. When a loaded magazine is inserted into the gun the belly of the BCG applies downward pressure onto the top cartridge in the magazine. When the charging handle is initially pulled back the BCG moves rearward with the handle which removes the belly of the BCG off of the top cartridge allowing the top cartridge to move upward due to magazine pressure. When the charging handle is released the BCG travels forward and is allowed to strip the top round from the magazine while positioning it in the bolt face. Then it proceeds forward to chamber the round and lock the bolt into the barrel extension. The round is then fired when the firing pin is struck by the hammer of the Fire Control Group. As the carrier is moved rearward by the gas that’s been applied to it from the combustion process the cam pin moves thru its travel in the carrier which unlocks the bolt from the barrel extension. As rearward movement continues the spent casing is extracted from the chamber. As the bolt moves within the carrier the gas rings travel within the ring bore that also contains two exhaust ports. When the rings pass the ports, the excessive gas and pressure is exhausted from within the BCG. During continued rearward movement the spent casing is ejected from the gun. The buffer spring and buffer play a big role in all of this also but that’s another story. It should be noted though that the buffer spring and buffer slow and return the BCG back forward into battery. As it does this the BCG picks up another round from the magazine and process is repeated each time the AR is fired. Simple, right? And it does all of this on its own thru the gas that is transferred into the gas key from the gas tube during the firing sequence. Do you now have a better understanding of why the BCG is the heart of the gun as well as one half of the engine?
Bolt Carrier Group Components
Let’s look at a breakdown on the major components that make up a bolt carrier group:
- Carrier- This is the external housing for the bolt and is attached to the gas key. It’s important to clean and check it regularly to ensure that the screws attaching it to the gas key do not come loose.
- Bolt- The bolt helps guide the round into the chamber, extracts the spent case, then ejects the spent cartridge once you’ve fired the gun. This is typically going to be made of 9310 or Carpenter 158 steel. With a preference of Carpenter 158 steel.
- Bolt Body
- Gas Rings
- Extractor spring
- Extractor spring insert and/or o-ring
- Ejector spring
- Bolt face
- Gas key- Also called the “bolt carrier key”, this essential part funnels gas from the gas tube into the bolt carrier. It should be properly torqued to specs, preferable sealed to the carrier, and staked to create a seal. Proper staking keeps screws from backing out.
- Firing pin- A firing pin is a component that strikes the primer of a cartridge, causing the gun to detonate.
- Cam Pin- The cam pin keeps the bolt inside the bolt carrier and keeps it in line, it keeps the firing pin in line, it “cams” the bolt into the locked position as the BCG finishes its forward movement into battery, and it provides many more functions that are crucial to the operation of the firearm.
- Firing Pin retaining pin
Different Types of Bolt Carrier Groups
There are a few different type of BCG’s within the AR platform. Some of these are AR-15 full-auto carriers, AR-15 semi-auto carriers, Mil-Spec carriers, AR-15 lightweight carriers, AR-10 and LR308 platform BCGS, Pistol Caliber Carbine (PCC) Blow Back Bolts, and Radial Delayed PCC Bolts. Let’s examine the common ones and break them down.
The Full-Auto “M16” BCG
The term “full-auto bolt carrier group” or “M16” is a bit misleading in that it doesn’t actually make your gun fully automatic. In order to do that, you’d also need a number of fully-auto components, including a sear and disconnector, along with your full-auto BCG. And not only are all of those parts difficult to find, very expensive, but acquiring them would probably put you in violation of a number of serious Federal laws as they are serialized parts that require special licensing.
To dispel a long time myth, you can use a full-auto BCG in semi -automatic AR-15. Having a full-auto carrier does not make it a “machine gun.” A full-auto M16 BCG is not considered a machinegun part by the BATFE, and there are no “special regulations” that apply to a M16 BCG vs the AR-15 BCG. This was backed up in an official statement from the BATFE:
“An M16 bolt carrier is simply a machinegun part and as such its domestic sale and possession is unregulated under the Federal firearms laws. It is not unlawful to utilize a M16 machinegun bolt carrier in a semiautomatic AR15 type rifle.”
More so, with a few exceptions, you should use a full-auto carrier over a semi-auto carrier, and this is due to operational reliability concerns of the AR with a semi-auto carrier in some circumstances. This is due to the longer lug on the back of the full-auto carrier which adds approximately 1oz to the overall carrier weight. This added weight can help the rifle cycle more smoothly and chamber rounds better when the rifle is fouled or dirty.
A full-auto BCG is also slightly longer at the rear than a semi-auto variant and features a lug. This little bit of added length is an aid when it comes to the BCG free travel. Excessive free travel can cause damage the lower receiver, gas key, carrier, and buffer detent. Another operational parameter that a full-auto BCG aids with is preloading the buffer and buffer spring. When the upper receiver is closed the back of the carrier should slightly push back on the buffer. This preloads the buffer and buffer spring, but more importantly it also removes the buffer from the buffer retaining pin. A short carrier can cause the buffer to rest on the buffer pin all of the time, and this can cause damage to the buffer retainer as well as to the face of the buffer.
Another benefit of the full-auto carrier is that they also tend to be more durable than semi-automatic ones. This is because all full-auto BCG’s have shrouded firing pins, which helps evenly distribute force throughout the carrier and to the hammer. Bolt carrier groups with unshrouded firing pins, on the other hand, experience more wear and tear and can cause your firing pin to break after excessive use.
The good thing about full-auto BCGs is that they’ve become quite prevalent in recent times, and this is what you will mostly find in the market.
The Semi-Automatic BCG
When it comes to price, semi-auto BCG’s are often the least expensive of the bolt carrier groups. While that doesn’t mean that semi-auto BCG’s are necessarily cheap and of poor quality, it does mean that they’ve been overshadowed by their full-automatic counterparts.
Just as the name suggests, semi-auto bolt carriers have been specifically designed to only support semi-auto fire. This was achieved by cutting off part of the bottom of the bolt carrier so that the bolt carrier wouldn’t engage the auto sear release, allowing the gun to fire fully automatic. While an understandable precaution set in place to prevent the use of automatic weapons, the problem with semi-automatic BCG’s is that they never were really necessary.
In fact, the vast majority of civilian AR-15 builds don’t have a sear release located in their trigger group, which means that regardless of whether your BCG is a semi or fully-automatic variant, it’s only capable of semi-auto fire. And for someone to actually convert their AR-15 into a fully-automatic rifle, they’d have to go out of their way to acquire parts that aren’t readily available and then modify their gun.
The Mil-Spec BCG
When it comes to firearms, “mil-spec” is a buzzword that’s often used to increase sales by associating products with the US Military. The important thing to remember before you buy any mil-spec part is that the term mil-spec means that the piece meets military specification.
It does not mean that the part is superior to other parts on the market.
With that said, it’s safe to say that mil-spec parts are of good quality. More often than not, they’re designed specifically for functionality and are constructed to withstand moderate to heavy use. In order for a BCG to be truly mil-spec, it has to be of the full-auto variety. Which makes sense, because military rifles like the M4 and the M16A2 are both capable of firing full auto.
Aside from that, mil-spec BCGs need to also meet the following other guidelines:
- High-pressure testing (HPT).
- A magnetic particle inspection (MPI).
- Carriers are made of 8620 steel.
- Bolts are made of Carpenter 158 steel
- Torqued and staked grade 8 gas key screws.
- Heat treatment, followed by shot peen.
- Interior of the bolt carrier and carrier key are chrome lined.
- The bolt and bolt carrier are coated with a matte black mil-spec finish.
The newest and most popular type of bolt carrier group on the market is the lightweight BCG’s, also known as low-mass bolt carrier groups. Like the name suggests, lightweight BCG’s weigh significantly less than their semi and full-auto counterparts. While this sounds appealing, under no circumstances should a low mass BCG be used in a defensive or duty rifle due to durability and operational reliability concerns.
To give an idea of just how light the low-mass BCG’s are, consider the following: your mil-spec bolt carrier group weights around 11.5 ounces in total, while a lightweight BCG averages around 5.75 ounces, which is half the weight. For competitive shooters and hobbyists looking to cut back on as much weight as possible, lightweight BCG’s are a great way to lose some unnecessary ounces.
Along with reducing the overall weight of your gun, lightweight BCG’s are sought after because they provide a smoother shooting experience with less muzzle movement and reduced recoil. The reason for this is simple: as you fire your gun, gas pressure from the fired round pushes the BCG back, cycling the ammo. The heavier the BCG weighs, the more recoil you’ll feel as a result. So when you fire with a lightweight BCG, you experience significantly less recoil because of its lower weight. The lower mass of lightweight BCG’s also helps them move faster, thus causing your gun to cycle through ammo more quickly than it would with a mil-spec or semi-auto BCG.
A lightweight BCG is designed for a tuned rifle and a builder who’s willing to toss an adjustable gas block on and really tune the gun system. The heavier standard BCG’s are better suited to running any kind of ammo through any kind of gas system. Typically when running a low mass BCG you also should run finely tuned ammunition which can only be achieved by handloading your own ammunition.
For the casual firearms enthusiast, the advantages of a lightweight BCG might not seem worth their hefty price tag, which can run between $200 and $500. But if you’re a competitive shooter in need of a high-performance rifle for your 3-Gun competitions, and you’re prepared to spend a little extra money building a gun finely tuned for your specifications, a lightweight BCG and an adjustable gas block may be for you.
Pistol Caliber Carbine Bolts
The latest AR platform craze appears to be Pistol Caliber Carbines (PCC). These are AR’s chambered in 9mm, .40 S&W, 45 ACP, 10mm, and a few other calibers. The 9mm PCC’s are big in both USPSA and IDPA shooting competitions.
The most common PCC is the blow back operating system. I’m not going to discuss the operation in this article, but due to the operating principle of blow back guns they require the use of a “bolt.” A PCC bolt looks similar to a BCG except that they do not have a true bolt, but instead have a flat face. PCC bolts are also a few ounces heavier than the full-auto BCG’s. Like a standard AR bolt, PCC bolts contain an extractor and ejector, as well as the firing pin.
An important note on PCC extractors. There are two types of extractors found in PCC bolts. There is an internal type extractor that looks very much like a 1911 extractor, as well as an external M16 style extractor which is very similar to what is found on a regular AR bolt. Internal style extractors are not as reliable as the external extractors and often require adjustment to reliable operation. These extractors are adjusted by bending them, just as you do to a 1911 style pistol extractor.
The belly of a PCC bolt is also cut just a bit different. The bottom of the bolt has a channel cut for the PCC ejector. Due to manufacturing tolerances as well as that there is no manufacturing standards for PCC’s, it is not uncommon to find that the ejector which is located in the lower receiver may not properly align in the bolt channel or it may rub on the bottom of the bolt. This is something that needs to be checked when installing a bolt.
The belly of a PCC bolt may also be ramped or flat. This affects what type of Fire Control Groups may be able to be used. A ramped belly looks very similar to that of a standard AR BCG and will allow for the use of most any Fire Control Group that is made for the AR platform. But there are still some that may not work properly. A flat belly PCC bolt requires the use of specific PCC Fire Control Groups and this is due to a specialized hammer that is required for proper operation. Ramped belly bolts are commonly found in the market.
Another thing to note with PCC bolts is that some have a removable weight in the tail of the bolt. Bolts that have this removable weight are designed to work with certain types of buffer assembly designs such as the JP silent captured buffer assembly. This is important to know if you are looking to buy a new bolt or are looking at assembling a PCC. You need to get a bolt that is compatible with your buffer system.
AR-10 and LR308 BCG
For the most part these are just a bigger version of the AR-15 full-auto BCG’s. The large caliber platform AR’s are not standardized unlike the AR-15 platform. What this means is that you have to select all of your components, including the BCG, for the platform you are working on. Some platforms are standardized while others are proprietary.
A few of the different types of platforms are:
DPMS (High and Low types)
DPMS Gen II
And other proprietary manufacturers….
Different Types of Material for Your BCG’s
Bolt carrier groups come in a lot of different types of material, and what they’re made of will determine how durable, reliable and expensive your part will cost. Let’s look at some of the common materials used to make BCG’s and their uses.
There’s a reason why steel is commonly used to make BCGs and other parts of the AR-15 and other guns. Steel is reliable, hardy enough to withstand heavy use, and it’s affordable.
Almost all steel BCG’s are built from 8620 steel, which is the same type of steel used in mil-spec BCG’s and is also the most common carrier material found in the market. The only real exceptions to this are Carpenter 158-steel carriers, as well as a range of lightweight stainless steel BCGs developed by JP made from 416 stainless steel. The latter is said to be better quality than their mil-spec counterparts.
Because of its lightness and strength, aluminum is commonly used for lightweight bolt carrier groups. In fact, aluminum is approximately 40% lighter than titanium, another popular lightweight BCG metal.
Because of its low mass, aluminum BCG’s reduce the recoil, making it easier for the shooter to make accurate follow-up shots. The downside to aluminum BCG’s is that they’re not able to take a lot of wear and tear. Even when the material has been given a hard anodized coat, excessive contact with heavy hammers can deform the metal over time. Because of this, it is recommend to use aluminum BCG’s for competition use only.
Titanium bolt carrier groups offer the lightweight advantage of aluminum with the durability and dependability of steel. Most manufacturers that make titanium BCG’s use 6AL-4V titanium alloy. This material is used extensively in racing and aerospace industries – including commercial aircraft like the Boeing 787.
While titanium isn’t as light as aluminum, it is roughly 40% lighter than 8620 steel. It’s also more resistant to corrosion and breakage. With that said, titanium is softer than steel, which makes it vulnerable to excessive friction and impact forces. To mitigate this, most manufacturers spray their titanium BCGs with a hardening finish like nickel boron to keep them protected.
Still, titanium is one of the best materials if you’re able to spend $300 or more on a BCG. Its versatility makes it the perfect material, regardless of whether your AR-15 is used for competitive shooting or just range shooting.
As the name suggests, hybrid BCG’s are made from one or more of these materials being mixed together with other metals. One of the more common hybrid materials is a mixture of steel, titanium, and aluminum.
Vella Industries Hybrid Titanium, Steel, Nickel Boron BCG
Generally speaking, most hybrid BCGs are designed to fulfill certain properties – like increased strength and reduced weight. For this reason, they’re usually some of the more expensive types of bolt carrier groups on the market. With that said, hybrid BCG’s are hard to classify and generalize due to the fact that there’s not a standard material that companies work with. For example, with steel, we know that most manufacturers use 8620. But with hybrid BCG’s, things aren’t as clear-cut.
In fact, you’re probably not going to get the same metal used uniformly throughout the part. Some parts of the BCG may be aluminum, whereas others could be titanium or steel.
For this reason, it’s best to look at hybrid BCG’s as individual products rather than its own unique grouping. For example, in the Vella Industries Hybrid BCG the carrier is made of hardened titanium, the firing pin is also titanium, the bolt is 9310 steel coated with Nickel Boron, and the gas key is hardened billet 4340 steel.
Bolt Material & Shot Peening
The mil-spec steel used in the bolt is Carpenter 158 steel. It is a very strong steel, but it is also expensive. Carpenter 158 steel is an alloy that was designed back in the day just for AR bolts. Some manufactures are using 9310 steel. Which is better? It depends who you ask. While most individuals will argue that 9310 steel is just as strong as Carpenter 158, 9310 steel bolts rely upon proper heat treatment for this strength. 9310 bolts have a documented history of breaking due to improper heat treating as the 9310 steel heat treat procedure requires very strict control in order to meet the specified surface and core hardness requirements.
One other thing to note about 9310 steel bolts. In my experiences 9310 bolts have a higher failure rate than normal if they have been coated in Nickle Boron (NIB). I’ve seen a few that have had catastrophic failure where bolt heads have broken completely off. I’ve also seen more coated 9310 bolts crack and break lugs on the bolt head. Just something to be aware of should you choose to you a 9310 bolt.
Many manufacturers also “shot peen” their bolts. This is a process that essentially makes the metal more resistant to cracks. During the shot peening process, tiny spheres known as “shot” are blasted at the bolt, with the shot acting as a tiny peening hammer that relieves stress from the metal.
Different Types of BCG Coating
These days, it’s not just titanium BCG’s that receive a special coat – most mid-to-high range BCG’s are given a coat for looks, strength, protection against heat, and various other properties. Some even claim to have increased lubricity properties. Here are some of the common types of coating that you’ll probably come across when shopping around for your next BCG.
Manganese Phosphate Coating
This is the mil-spec coating for AR-15 BCG’s. While it used to be by far the most common coating you would see, that is not so much the case anymore. It’s economical, durable, and highly corrosion resistance. Since the surface is more porous than some of the more exotic coatings, it’s harder to clean and this is why it isn’t as prominent in the market as it once was. Stronger than nickel boron and other coatings,, phosphate gives your BCG a hard, protective layer so think that it actually increases the overall thickness of your bolt carrier group. Due to its durability, phosphate is a good choice for defensive and duty rifles.
Nitride Coating (Nitrocarburizing, Black nitride, Melonite, QPQ, Tennifer)
Nitride finish, commonly known by the brand names of Melonite or Tennifer, isn’t technically a coating. It’s a chemical treatment that results in a hardened black surface that is both durable and highly corrosion resistant. Black Nitride is a process commonly referred to anonymously with SBN (Salt bath nitride) and sometimes QPQ (quench polish quench).
The coating involves high temperatures of nitrocarburizing. When done correctly, you’ll have a very tough, no-frills BCG that is easier to clean than phosphate coatings and reduces friction.
A downfall to nitride coating is that it is thin. What most individuals don’t realize is that most (not all, but most) BCG’s are made to TDP specs which calls for a phosphate coating and chrome lining. Therefore when nitride and some other coatings are applied it leaves the bores of the carrier and gas key a little bit undersize as chrome plating is thicker. Dependent upon tolerances, this can make for a BCG that is not gas efficient and could create concerns with an AR if there are other gas inefficient areas due to gas leakage. When I gauge BCG’s and BCG components I find a few common areas of concern with nitride BCG’s.
Nickel Boron/ Electroless Nickel-Based Coatings
Albeit expensive, nickel boron is one of the most commonly found and used BCG coatings. Mainly due to its looks and ability to clean. From my experiences I have seen this happen sooner than later. A caveat here though is that NIB will eventually turn a dull gray in color due to staining. The electroless nickel coating process results in a thin surface layer that is harder than the underlying metal, with improved dry lubricant properties. This finish comes in many varieties, with the most common being nickel boron.
Dozens of other variants exist, such as Nickel Boron Nitride, EXO, and NP3 . NP3 is an electroless nickel-based finish for steel, stainless steel and aluminum alloys. This finish works by depositing PTFE (polytetrafluoroethylene, aka Teflon) particles, along with electroless nickel. The main benefits of this coating are high lubricity and a low friction co-efficient. This translates to possibly less cleaning needed for smooth function, and perhaps less wear on the coated parts.
NIB coating also helps to make your BCG more resistant to corrosion. Which it does well if it doesn’t flake. Flaking of the NIB coating is another well documented concern. These varieties tend to enhance the hardness and/or lubricity of the coating. The electroless nickel coating is known to wear off with time. This affect can be reduced by sticking to high quality parts and enhanced coating varieties, but it is still worth noting.
NIB coating is thick and I have seen many BCG related concerns due to this coating. I have had new BCG’s come in to the shop that were out of spec due to the coating. Some had feeding issues while an LR308 NIB coated BCG was so out of spec that it wouldn’t clear a magazine and would catch on the back side of it. It is also common to find NIB coated carriers to be coated separate from the gas key. This gets the coating into the threads of the gas key holes and provides for false torque readings when the gas key screws are being torqued. This results in a gas key that leaks and causes cycling issues. I’ve had a number of these through the shop. Some manufacturers NIB coat all of the BCG parts. You do not want the extractor to be coated. The NIB coating can reduce surface tension between the extractor hook and the case head resulting in the extractor slipping off of the case and the case being left in the chamber.
Of all of the coated BCG’s that I have personally used or have experience with as they come into the shop, NIB coating is by far the worst and one that I would stay away from.
Vapor Deposition Coatings (IonBond, Titanium Nitride/TiN, PVD, CVD)
Commonly referred to as IonBond, CVD, and PVD coatings, this category of coatings is characterized by specific manufacturing process known as vapor deposition. The resulting coating is thin, extremely hard, and slippery – reducing the need for lubricant and speeding up cleaning. These coatings are touted to have the best qualities that phosphate and nickel boron have to offer. The two coatings that are commonly found are Ion Bonding and TiN.
Ion bonding provides the hardness of phosphate with nickel boron’s protection against friction. This means that your BCG is coated with a hard protectant that keeps it better protected against corrosion and heat. The general consensus is that a nickel boron coating provides a smoother finish, but ion bonding is a close second.
TiN coatings give the smoothness of nickel boron, but with a sleek, gold finish. For this reason, this type of coating is incredibly popular among hobbyists who enjoy customizing their gun for aesthetic reasons. However, when it comes to performance, TiN coatings don’t serve much of a purpose over nickel boron other than looking nice and it comes at a price.
This is newer technology, but the favorable characteristics of vapor deposition coatings have made this one the fastest-growing category of BCG coatings. As an added bonus, the manufacturer’s ability to customize the color of the coating has led to some exotic new BCG colors hitting the market.
The big downside with this type of coating is cost – often 2 or 3 times the price of a phosphate BCG.
Chrome possess superior hardness and reduced porosity vs a phosphate BCG, making it smooth and easy to clean. There are still chrome BCG’s on the market, but they have lost some popularity with the rise of newer coating technologies. There are very few manufacturers doing chrome anymore and for those that are the lead time can be substantial. The big downsides of chrome are that is expensive, and the finish can become brittle and flake off with time.
Putting Your BCG through the Stress Test
Bolt carrier groups go through a lot of stress from frequent usage, which is why people are willing to spend extra money on high-quality metals and different types of coatings. To guarantee that BCG’s are as durable and reliable as the manufacturers say they are, these parts are often tested to see just how well they’re able to perform.
The Magnetic Particle Inspection
One way that bolts are assessed is through a magnetic particle inspection. If you recall from the above, MPI is part of the mil-spec specification. This test scans the surface (or slightly below the surface) for imperfections and cracks that could compromise the quality of the BCG. The process works by magnetizing the BCG and then applying iron particles to its surface. Any areas on the BCG with small cracks and scratches should pull these particles towards them, thus revealing the defects.
High-pressure tests are designed to place the bolt under extreme pressures by firing a proof round. As an example, a 5.56 NATO M197 proof round generates 70,000 psi. The purpose of this type of testing is to make sure that the part is strong enough to be used in the field. However, this type of test is also the source of a lot of criticism, as some experts believe that high-pressure testing permanently weakens the bolt, effectively reducing its lifespan in the process.
A Caveat About Testing
When BCG’s and bolts are tested, they undergo one of the following types of testing: batch testing or individual testing. Like the name suggests, batch testing assesses the quality and craftsmanship of an entire batch of BCGs. The benefit of this kind of testing is that it has lower labor costs, which means a cheaper price tag for you. The downside, however, is that there’s always the possibility that each BCG isn’t getting the attention they deserve, which means that some defective pieces could make it through the testing phase.
With individual testing, on the other hand, pieces are given a rigorous testing on a one-on-one basis. This gives companies more time to thoroughly inspect the product, looking for minor defects that could’ve been overlooked in batch tests. The downside is that individual testing requires more labor, which means that the individually-tested pieces are more expensive as a result.
As was discussed earlier there are different styles of BCG’s and bolts for different AR platforms. You need to aware of this and what their differences are if you are looking to replace yours or if you are assembling an AR. Some may not be interchangeable or work with other components in your AR.
The AR was originally designed for the .223 Remington and then adapted to the 5.56 NATO cartridge. Thankfully these use the same case head and bolts. But the same can’t be said for other calibers in the AR-15 platform. With the exception of the 300 Blk. Other calibers change the feed angles, stresses, and operation of the AR platform, but more importantly they also require the use of different bolt head diameters and depths. What does this matter? Well certain calibers such as the 6.5 Grendel and 450 Bushmaster have very little bolt webbing thickness left on the bolt and therefore are prone to breaking bolts. This doesn’t just apply only to these, but it is an example. Then there are also extractor breaking concerns on multiple calibers such as the 6.5 Grendel, 7.63×39, and others. The changes in the bolt face positions the case head of certain calibers deeper or shorter which affects the extractor position and results in pronounced stress on the extractor causing it to break. Some calibers you will see with chamber designations of I or II, etc. and this combined with changes in headspace are a couple of the main reasons for this.
This is just something to be aware of if you shoot calibers other than .223, 5.56, or 300 Blk. There are some manufacturers out there that make improved bolts and other components for certain calibers, but they come with a price tag. As an example, there is an enhanced 6.5 Grendel bolt but the last time I saw the price on one of these it was basically the same price as a budget BCG. Around $100.00.
What Does It All Mean?
A bolt carrier group is the heart of an AR-15 rifle. It has a gas impingement design which performs firing, ejection, and cycling of rounds consecutively. When it comes to bolt carrier groups, there’s a lot of information to digest before choosing the right part for you. Factors such as materials, weight, coating, staking, inner lining, and testing must be considered before choosing a BCG.
The good news is that it doesn’t have to be as complicated as it looks. A full-auto BCG is more preferred these days due to its versatile nature and shrouded firing pin. Low-mass BCGs, on the other hand, have their own benefits such as lesser recoil, but they largely suit competitive shooters.
The BRD Gun Works Way
If you’ve read this far you may be asking what it is that I use. First off it is dependent upon caliber and platform as these can be limiting factors. Next is, what is the purpose of the AR? As an example, for 5.56 NATO/ 300Blk Carpenter 158 bolts are used. With other calibers this is not an option and the only option may be a 9310 steel bolt. I prefer phosphate BCG’s but these are not always available with my preferred other components. In this case, nitride coated BCG’s are my next option. I also use phosphate gas keys for their tight tolerances and chrome lining.
In all fairness though, I hand assemble all of my personal BCG’s as well as the BRD Enhanced BCG’s. After a lot of research and testing I have selected a manufacturer for the carriers and bolts, while selecting another manufacturer for the gas keys. I still gauge all of the components of the BCG and reject anything that I may find out of spec. As previously stated, I prefer phosphate for my personal BCG’s, but most people like a coated BCG so the only coated one offered is a nitride one.
There are many other enhanced parts and labor that go into a 5.56 NATO/300 Blk BRD Enhanced BCG and they are:
- M16 8620 Tool Steel Carrier
- Carrier is Nitride Finish
- Carpenter 158 Steel Bolt
- Heat Treated
- Shot Peened
- HPT Bolt – High Pressure Tested
- MPI Bolt – Magnetic Particle Inspected
- S7 Tool Steel Extractor
- Sprinco Extractor Spring & Insert
- Sprinco Ejector Spring
- 8620 Phosphate w/ Hard Chrome Internal Gas Key Hardened to USGI Specifications
- Gas Key Face is Trued
- OCKS (Optimized Carrier Key Screws) Fasteners
- Gas Key Screws Hand Staked
- Gas Key Sealed to Carrier Using 620 Loctite
- 17-4 Stainless Firing Pin
- 8620 Cam Pin
- Machined to USGI Specifications
- Bolt Comes Pre-Lubed with Cherrybalmz BRB
- Bolt carrier group assembly has been gauged and inspected
- Ejector has been chamfered and polished
- Extractor has been polished
- Firing pin hole and ejector tunnel opening has been de-burred
- Each bolt carrier group is hand assembled
I hope that this article has been informative for you, that you have picked up a thing or two that you didn’t know, and that it will help the next time that you have to select a BCG.