Remember that silly politician who went on TV to embarrass himself with incorrect firearm terminology… “Which one?” you might ask. We’re talking about former California Senator Kevin de León who previously claimed that “the heart of an AR-15 is the lower receiver” when making remarks about 80 percent lower receivers during a press conference in 2014.
We’d, along with many others in this industry, would argue that the bolt carrier group (BCG) is the actual heart and engine of an AR-15 (or any semi-automatic rifle platform for that matter). There are so many types of BCG’s that we decided to help simplify the research and choosing process for you.
When looking into BCG’s for the first time, you might feel overwhelmed by the barrage of new terms and potential buzzwords. No worries, we’re here to help you with that:
BCG – Bolt carrier group. It’s important to understand the distinction between a bolt carrier, the bolt it carries, and how these two main components together are referred to as a bolt carrier group.
“Mil-spec” – 100% a buzzword as much as “all natural,” “cage-free eggs” or “organic.” All mil-spec means is that it meets the minimum requirements to fulfill a military contract standard.
“MPI” Magnetic Particle Inspection – This process is meant to look for cracks in metal that the naked eye can’t see. It’s done by applying a special solution with magnetic particles to the BCG and then hit by an electrical charge. The BCG is then inspected via a UV light, which can visibly identify imperfections and cracks in the metal due to the concentration of dye in certain areas.
“HPT” High Pressure Tested – Another form of quality control; this is where a live round with excess gunpowder is fired in a barrel assembly to test a BCG’s integrity. After firing, the BCG is removed from the assembly and inspected for any failures.
Shot-peening – A metal finishing process that involves shooting tiny glass, ceramic and metallic materials onto a BCG’s surface at high intensity which helps to significantly increase metal fatigue life.
Chrome Lining – Also known as chrome plating, all mil-spec bolt carriers should have this metal finishing on the interior of the BCG to help prevent corrosion and increase durability. The metal coating is applied via electroplating on the BCG’s interior metal surface.
How many types of BCG’s are there?
AR-15 Bolt Carrier – We’ll get into all the different types of BCG’s and the various types of coatings which offer different benefits but generally speaking, the AR-15 bolt carrier can be differentiated by its shape. Its rear actually has a little less materia compared to an M16 bolt carrier due to the M16 having full auto firing capability and a different trigger assembly.
M16 Bolt Carrier – The M16 bolt carrier has more material in its rear compared to the AR-15 bolt carrier to accommodate for trigger assemblies with auto sears which allow for full auto fire. The AR-15 and M16 BCG are interchangeable with any standard rifle.
Low Mass Bolt Carrier Groups – Also known in some layman’s terms as “half-circle” bolt carriers due to their shape, low mass BCG’s are essentially shaven down bolt carriers designed to reduce overall weight off the gun. Lighter mass going back and forth equals less recoil equals easier to stay on target.
Steel Bolt Carrier Groups – The choice of our military’s M16 and M4 BCG’s have all been made by steel because they are tough, durable, heat resistant and able to handle sustained abuse. Though steel is the most common material for BCG’s, different types of steel are used for individual components.
Titanium Bolt Carrier Groups – Titanium BCG’s can double or even triple the cost of a standard steel BCG because titanium is not only lighter but also stronger. It’s more resistant to heat and pressure than steel. They also come in more attractive coatings in a variety of colors which may look shinier and feel smoother.
Full-Auto Bolt Carrier Groups – As mentioned earlier, this is essentially an “M16 BCG” which is designed to trip an auto sear so that an M16 or M4 can fire full auto. It is completely legal to have one of these for your AR15 as it will not fire in full auto unless you install an auto sear (which is illegal/highly regulated).
Adjustable Carrier Group – This is actually really cool — as one of the newest kids on the block, this BCG allows you to switch between different positions to help you fine tune the BCG’s vent settings. By doing so, you get to adjust the BCG for recoil reduction.
Hybrid Design – From our brief research, what we’ve found is that BCG’s that are titled a “hybrid design” usually mean they’re made with a nicer coating and some sort of “extra” feature. Perhaps the BCG is compatible with multiple style builds. I.e. an AR9 BCG that works with both Colt and Glock AR15 style builds. Or there could be an extra tilt when the BCG cycles that helps mitigate some recoil.
How important is the material type?
At the end of the day, any reputable BCG-making brand shouldn’t do you wrong, but once you start shooting tens of thousands of rounds through your rifle, that may be where you start to see it letting up. A lot of people really do shoot that much, and could easily do so Pre-COVID-19. We’re actually starting to see ammo prices drop slightly as well so it wouldn’t hurt to consider the different BCG types and their varying quality out.
Tensile strength – The measure of maximum stress that a BCG can withstand without being elongated, pulled, or stretched.
Yield strength – The force exerted at which the BCG would permanently deform.
8620 Steel – This is what all mil-spec BCG’s are made of and to help give you a better idea of the varying steel’s quality in strength we’ll be showing you their tensile and yield strength measured in MPa which is a megapascal pressure unit. (1 MPa = 10 bar) In our case here, MPa is used to describe the pressure ranges that a BCG can handle.
Tensile Strength: 640 MPa
Yield Strength: 360 MPa
9310 Steel – Keep in mind: even if you have a “mil-spec” BCG, most of the bolt components will actually be made by 9310 steel or Carpenter 158 steel. The bolt components would include the bolt body, gas rings, lugs, extractor, ejector, and the bolt face.
Tensile Strength: 910 MPa
Yield Strength: 570 MPa
Carpenter No. 158 Steel –
Tensile Strength: 1103 MPa
Yield Strength: 931 MPa
S7 Tool Steel –
Tensile Strength: 2030 MPa
Yield Strength: 1520 MPa
What are the different types of finish?
Phosphate Coating – Not to be confused with nitride coating, phosphate coatings come in a matte black finish which is much rougher to touch than nitride. Because it is more porous it retains lube much better in comparison to nitride. All mil-spec BCG’s without any fancy coatings come in phosphate.
Nitride Coating – Is short for SBN which is salt bath nitride — an electrochemical process that creates a slick and hard coating that is durable and extremely resistant to corrosion. Oftentimes we’ll see handgun barrels and slides coated in some form or color nitride.
Melonite Coating – The same as nitride coating, simply a brand name for the treatment process.
Nickel Boron Coating (NiB)– A slick and shiny alloy that provides a metallic coating which dramatically reduces friction as the BCG cycles back and forth (also increases performance). Because the BCG runs so smooth it doesn’t require much lubrication if any at all. If you do like applying lube, it wipes off and cleans very easily without using any hard cleaning chemicals. Typically, NiB BCG’s will be a bit more expensive.
DLC Coating – Important note: on some BCG’s you might find a combination of different coatings used for different parts. For example, a titanium carrier could be coated in DLC, while the bolt itself was coated in NiB. DLC stands for diamond like carbon which of course has an extremely high wear resistance with a low coefficient of friction. The largest added benefit with DLC is increased heat dissipation which makes a BCG more resistant to thermal stress.
Ok, so who makes the BCG’s?
Almost all BCG selling and manufacturing company’s will buy their bolt carriers from one of three companies: Toolcraft and Micro-best. Toolcraft makes up most of the mil-spec BCG’s on the market while Microbest accounts for the rest.
Azimuth Technology is not as well-known or mentioned as often but they are the largest OEM manufacturer in the country and they have a hand in just about everything that is made in OEM specs. That would, of course, include BCG’s.
Companies will often buy bolt carriers from either of these companies and possibly buy the bolt itself from somewhere else if they don’t manufacture it themselves. Then, they’ll simply package the whole thing together and slap their brand name onto the BCG.
The honest truth is that there isn’t a huge difference between the two brands (Toolcraft and Micro-best). For 99% of end users, Toolcraft will be certainly good enough. However, Microbest will make the difference for competitive shooters and those in duty functions such as military or law enforcement. If it means anything to you; all the BCG’s used in AR-15’s made by Sons of Liberty, Daniel Defense, Triarch, Sionics Weapons Systems, Type A Rifle Company, and SLR Rifle Works all use Microbest BCG’s.
What brands do we recommend?
As mentioned before, any reputable brand selling decent BCG’s are not going to be an issue for most shooters. So, we will show you the very best BCG’s money can buy:
The Lewis Machine and Tool Defense Enhanced BCG is the toughest, one of the most expensive BCG’s to date. You’d be very hard pressed to find any record of this BCG failing. It was designed to work in full-auto, suppressed, and short barrel weapon systems.
The Knight’s Armament Sandcutter BCG comes with grooves machined into the carrier which allows for more free movement and operation in extremely adverse conditions. The concept of this design was based on the sand cutting grooves which are similarly in older FN FAL carriers. Supposedly, the FAL was having a hard time in sandy environment so they came out with a carrier that are now referred to as “sand cuts” which would remove sand from the BCG during cycling to ensure more reliable performance. Because of the KAC Sandcutter’s chrome finish, it looks more like a showpiece but make no mistake it will perform beyond expectations in the harshest of environments.
The Lantac Enhanced Bolt Carrier Group is just as tough of a competitor to the aforementioned BCG’s. Their biggest claim to fame is the cam head pin which is rounded in case-hardened steel which wears less in the runway within the upper receiver. They also use redirected gas ports going forward instead of backwards keeping gas out of your face and helps a lot with reducing perceived felt recoil. Also it looks dope, and you can flex on the poors.