If you are building your first rifle, or even your tenth rifle, you may think the bolt carrier group is one of the easier choices you’ll have to make.
If you think so, go to Brownells and search ‘bolt carrier groups.’
How many did you see? More importantly, how many different types did you see? Quite a few I’d imagine.
Choosing your bolt carrier group can be a confusing task, with phrases like Full-Auto (FA), or the variety of material types, and acronyms like MPI.
Do not fear!
We are here to help you decode the mysterious language of bolt carrier groups. Things can get a little confusing with all the terminology and differing opinions from so called experts.
But…if you can’t wait, here’s our picks for the best BCGs:
- Mil-Spec BCG: Bravo Company M16 BCG
- Low-Mass BCG: JP Low Mass BCG
- Steel BCG: JP Full Mass BCG
- Titanium BCG: V Seven Titanium BCG
- Nickel Boron (NiB) BCG: WMD NiB BCG
- Titanium Nitride (TiN) BCG: F1 TiN BCG
Semi-Auto Bolt Carrier Groups
We’re going to start by looking at some of the terminology you’ll see thrown around regarding bolt carrier groups, and then we’re going to dive into my favorite BCGs from each category. Let’s get to it.
Note: If you aren’t totally sure where your bolt carrier group fits in the overall scheme of things, check out our AR-15 Upper Building Guide to make sure you know exactly where everything goes.
Alright, so the first type of bolt carrier group gets a lot of undeserved hate. Semi-auto carriers have fallen out of vogue and are basically treated with the same respect as commercial buffer tubes and stocks….even though they’re basically the same (so are commercial-spec buffers and stocks).
This isn’t fair, and really it’s just because “full-auto bolt carrier group” sounds cooler than “semi-auto bolt carrier group.”
This has largely lead people to dismiss semi-auto bolt carrier groups. If you are one of the few people out there with a full-auto M16 then yeah, it makes sense to dismiss semi-auto BCGs.
However, for the rest of us plebs, the semi-auto isn’t a bad choice.
Semi-auto bolt carrier groups have a small advantage of being slightly lighter in weight than FA carriers. Semi-auto carriers have a cutback rear portion and this where you lose your weight.
You can also use a lighter buffer due to the lighter carrier which results in even more weight savings.
The only thing that will be heavier is your wallet. Semi-auto bolt carrier groups are quite a bit cheaper than your standard full-auto carrier.
You can easily find semi-auto carriers well below 100 bucks.
Full-Auto Bolt Carrier Group
Sadly a full auto bolt carrier does not make your gun fully automatic. It needs quite a few more parts for this to happen.
The full auto bolt carrier group is basically the standard carrier these days. Almost every manufacturer who produces a rifle uses full auto bolt carrier groups. The vast majority of unique and interesting bolt carrier groups are full auto as well.
Full auto bolt carrier groups are a little longer on the back end, and you know what they say…fat bottom girls make the world go round.
The slightly longer and heavier bolt carrier group has an additional lug at the rear of bolt carrier group.
On a full auto AR-15 or M16, this rear lug pushes the sear release downwards. This allows full auto (or burst) fire. In a semi-automatic rifle, this has zero effect on how the gun functions. It does result in a bolt carrier group that is about an ounce heavier and requires the use of a heavier buffer to ensure it functions reliably.
A standard full auto bolt carrier groups goes for about a 100 bucks on average, so your wallet is left a little lighter. The best reason to own a full auto bolt carrier is the tiny bit of protest you can stage inside your rifle, for whatever that’s worth.
It’s also likely to be the most common you’ll find with the widest variety of options.
Mil-Spec Bolt Carrier Groups
Mil-Spec is a bit of a buzzword in the AR community. Mil-Spec generally means the rifle is built to pass military specifications. No semi-automatic AR-15 is truly Mil-Spec due to the lack of a non-semi-auto fire control group.
However, parts can be built to mimic the military specifications. Mil-spec doesn’t mean greatest ever, or worst, it’s just in spec to a military contract. The Mil-Spec requirements for a Bolt carrier group is pretty simple.
The guarantee that Mil-Spec offers is it’s quality enough to be used by the military. Going below Mil-Spec is essentially heresy in the AR world…but really it’s just a preference beyond making sure all your parts match up.
According to Brownells, here is the Mil-Spec of an M16/M4 bolt carrier group.
The Bolt is machined from Carpenter 158 Steel and is
- High pressure tested
- Magnetic Particle Inspected
- Heat treated and shot peened
- Bolt carrier machined 8620 steel
- Chrome lined interior bolt carrier key and bolt carrier
- Full auto carrier
- Coated with matte black mil-spec finish
Low-Mass / Lightweight Bolt Carrier Groups
Low-mass or lightweight bolt carrier groups are the newest type of bolt carrier group on the market. They are honestly one of the more interesting developments in the AR-15 world. We also have a whole guide just for choosing a lightweight bolt carrier group, so be sure to check that out if you want to know more.
A low-mass BCG serves two purposes. If you are following the current fad of super lightweight AR-15s you want to trim weight basically everywhere. You can cut a few ounces off through using low-mass bolt carrier.
A Mil-Spec BCG is about 11.5 ounces, but you can go as light as 5.74 ounces with the right low mass BCG. The primary advantage of this is to reduce recoil.
Since the bolt carrier group is shot backward towards the shooter the weight of it affects recoil.
The more mass you have coming rearward the more recoil you’ll feel. Any reduction in recoil and firearm movement, in general, helps you stay on or get back on target as you fire. A low mass bcg moves faster which could increase your rate of fire, but skill makes a bigger difference in cyclic rate than bolt weight in a semi-auto gun.
Now you may be thinking, this is genius! Why do regular bolt carrier groups still exist?
Well to put it simply 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. The heavier standard BCGs are better suited to running any kind of ammo through any kind of gas system.
So, if you’re building a SHTF, zombie-apocalypse-proof, rifle…go with a regular BCG. Building a performance-tuned competition rifle with an adjustable gas block? Go with a light-weight BCG.
Recommended Low-Mass BCG: JP ENTERPRISES – AR-15 LOW MASS BOLT CARRIER ASSEMBLY ($214.99)
Now that that’s all settled, let’s talk about materials.
Steel Bolt Carrier Groups
Good old fashioned steel is a reliable and durable option for building AR-15 BCGs. It’s the classic choice in construction for a number of reasons. It’s tough, heat resistant and built to withstand abuse.
More importantly for most casual users, it’s also pretty cheap and easy to manufacturer.
Recommended Steel BCG: JP Enterprises Full Mass Bolt Carrier Group ($351.99)
It’s far from fancy, and far from sexy, but it works. Steel BCGs are suited for duty and defense use and are used by the military and police.
Aluminum Bolt Carrier Groups
Aluminum is a material commonly used to make low mass bolt carrier groups. Aluminum is a light and decently strong material that can give a pretty good BCG for competition use. However, they aren’t meant for do or die duty use.
They are really only designed for guns with adjustable gas systems. They are more costly than steel and will wear out faster. To make them last longer it’s best to use a light hammer, and to ensure the hammer doesn’t have sharp edges that contact the carrier.
Recommended Aluminum BCG: JP Enterprises Low Mass Aluminum BCG ($322.99)
Titanium Bolt Carrier Groups
What if you could have the best of both worlds when it comes to steel and aluminum BCGs? Well, it turns out you can if you are willing to pay for it.
Titanium BCGs can’t get as light as aluminum on average, but are still way lighter than steel.
Titanium BCGs are also way stronger than both aluminum and steel BCGs. Titanium shrugs off heat, pressure, and dirty words better than any other competing material. Here the big problem, are you willing to pay for it?
I’m not joking that when researching this article I ran across the words, “Financing available”, a few more times than I’d like.
These BCGs are typically sold for over 300 dollars.
Recommended Titanium Bolt Carrier Group: V Seven AR-15 Titanium BCG Ionbond Finish
Hybrid Bolt Carrier Groups
There are a few companies that produce BCG’s from a number of different materials and often mix titanium, steel, and aluminum.
These BCG’s are rare and expensive. The mixture of materials gives them certain strengths in different areas and trims weight when possible.
It’s hard to classify these carriers without going through them one by one because they each use different materials in different parts of the bolt. So while one may have an aluminum rear body, another may have a titanium rear body. It’s impossible to group them together by features.
The Coating of a BCG affects how it runs, how easy it is to clean, and it’s ability to resist heat and corrosion. Different materials can utilize different coatings, but the three most popular and common are the following.
Nickel Boron Coated BCGs
Nickel Boron is a material that’s applied via a plating process.
Nickel Boron plating occurs through an autocatalytic reaction. Here’s what that means if you aren’t a chemistry major.
Short version: it makes for a good BCG coating.
Nickel Boron BCGs are very resistant to corrosion and require less lube to run smoothly. Just handling one allows you to experience the near-frictionless nature of nickel boron. What’s really cool is that cleaning it is as easy as wiping it down with a rag.
Nickel Boron can appear to be both gold or silver, which let’s be honest here, looks cool. Looking cool is half the battle.
Our go-to is the WMD NiB BCG ($175) which definitely has made our cleaning sessions much easier.
Phosphate Coated Bolt Carrier Groups
Phosphate coating is what most bolt carrier groups are coated with. This matte black or gray finish is created by tossing a bolt carrier into a tub of phosphoric acid with manganese and is heated to about 200 degrees American, err…Fahrenheit.
This creates a very tough coating that is layered on thick enough to increase the overall thickness of the BCG. Phosphate coating is stronger than Nickel boron and more resistant to heat and wear. However, they have more friction and foul easier. It’s also a bit cheaper versus other coatings. And what you’ll find on most “regular” BCGs.
Ion Bonded Bolt Carrier Groups
Ion bonded finishes are insanely durable and hard, which results in a very high level of corrosion and heat resistance, as well as a long lasting finish. Ion Bonded BCGs mix the good qualities of both phosphate and nickel boron.
Ion bonded coatings are tough and strong like phosphate coatings, and remain slick and near friction free. Nickel Boron coatings will result in the smoothest finish, but Ion bonded BCGs aren’t far behind. Ion bonded is a great compromise if you don’t mind the increased cost.
Titanium Nitride Coated Bolt Carrier Groups
TiN, or Titanium nitride coatings are popular because they look awesome. They are a very shiny gold in color, and the coating is as smooth and slick as nickel boron. The coating is also more durable than nickel boron. If that tickles your fancy…check out F1’s TiN BCG ($250).
It’s also quite costly and doesn’t result in substantial performance improvements past being tougher than nickel boron, so go for it if you want, just make sure.
Does Your BCG Pass the Test(s)?
Bolt carrier groups are often tested to prove their worth and as a quality control measure. We mentioned Mil-Spec BCG’s are tested, but so are most other BCGs. Testing ensures the product is strong enough not to fall apart after a little range time.
Magnetic Particle Inspected
Magnetic particle inspection is a nondestructive testing that looks for defects in the surface and slightly below the surface of a BCG. On a BCG magnetic particle inspection is looking for small cracks and imperfections that could result in major issues down the road.
The BCG is first magnetized, and then iron particles are applied to the surface of the BCG. The ion particles will be attracted to any leakage fields i.e. cracks and scratches. This provides a simple, and cheap way to visually make sure the BCG is defect free.
High Pressure Tested
High-pressure testing is quite controversial. The idea is that you fire a high-pressure test through the BCG to ensure it’s tough enough to withstand the pressure from a live round.
Sounds great right?
Well, many folks who are smarter than me claim this type of testing is destructive. It shortens the life of a BCG by submitting it to pressures that the BCG will never run into under normal use.
Most proclaim this testing is outdated and not worth the risk of future issues with a BCG. You’ll have to decide how you feel on that one.
Batch Vs Individual Testing
These types of tests are pretty self-explanatory. Batch testing means a group of BCGs are all tested at one time. This lowers labor costs, which are then passed to the consumer. The concern here is that each BCG doesn’t get the attention it deserves.
Individual testing means each BCG is tested one at a time. This is more labor intensive and obviously, adds some costs to the BCG. However, fans of this method like the idea their BCG got the special attention it really needs to succeed in college…. I mean in your gun.
Don’t Get Carried Away
A good pun is its own reword. Anyway, yes there are lots of different types of BCGs, made from different materials, at different weights, and they serve different purposes, but it isn’t as confusing as it looks. If you want a basic AR-15 just go with a simple mil-spec, or semi auto bolt carrier group.
It’s hard to go wrong there. Building a comp rifle you may want to go with a low-mass bolt. Building a tricked out rifle because money is no object? Go big with a titanium model. Also, build me a rifle.
What kind of BCG do you run? What kind would you like to see more info on? Let us know below.