The AR-15 bolt carrier group, or BCG, is a critical and often misunderstood piece of the AR-15/M16 upper receiver. Here I will give a brief breakdown of the BCG’s components, how it works, problems to look for and what to consider when buying.
The image below gives a nice breakdown of the parts in a typical BCG.
We won’t get into a lot of detail here, but this is a good illustration of how the BCG operates when an AR-15 type rifle is fired:
When the round is fired, the redirected gas from the cartridge is funneled back into the bolt carrier. The gas fills the chamber that’s created by the bolt’s gas rings and the bolt carrier and forces the bolt carrier back against the recoil spring.
As the bolt carrier moves rearward, it acts on the cam pin, which twists the bolt. This unlocks it from the chamber allowing the BCG to cycle rearward.
As you can see, the BCG itself has a number of parts and there’s a lot going on every time the rifle goes bang. Now that we have a basic understanding of the parts and how they work, we can talk about some common problems you might run into with the parts in your bolt carrier group.
Loose Gas key
Gas keys can come loose. The screws are (or should be) staked in place. If that’s done correctly, it will take a lot for the key to loosen…but it can still happen.
A loose key creates a poor seal so gas can escape. Depending on the degree of looseness, this can cause cycling issues due to a leaky, under-gassed system. That can mean failures to eject and short-cycling of the bolt.
My first issued M4 had a loose gas key. It was loose for quite a while. At the time I didn’t know any better, so I figure that’s just how they were. I eventually learned it was a problem and got it fixed, but I was lucky…I never had a malfunction due to the loose key. You may not be so lucky.
The internet is full of stories about Bushmasters and the malfunctions caused by their loose gas keys, but those were in the old days. If you have one, however, they’re not hard to fix. Re-tighten the screws and re-stake them. If you’re absolutely sure you’ll never ever want to remove the gas key, you can even use JB Weld or some other heat-resistant epoxy to fix them down. Red Loctite may work for a while, but it isn’t heat-resistant.
Staking works just fine, though. The image above shows two AR-15 bolt carriers — a Bushmaster on the left and a Colt on the right — both with properly staked screws. If you don’t want to worry about loose screws, a BCG design like this one from Voodoo Innovations eliminates the problem completely.
The gas rings create a seal between the bolt and the bolt carrier. This tight seal is what contains the gasses and drives the whole system. If those rings get damaged or worn to a point where gasses escape, malfunctions like those listed above can occur.
Unless the rings are obviously damaged, you can confirm the seal on your bolt is good by standing the BCG on the bolt face like in the image below.
If the rings make adequate contact with the bolt carrier and form a good seal, it will stand tall. You should check this even with brand new rifles.
Especially check it if the BCG is made from mismatched parts. The BCG on the left above is a Spike’s Tactical complete bolt carrier group with roughly 1000 rounds through it. It’s working just fine. The BCG on the right is brand new with a JP bolt and Rainer bolt carrier. It failed the test, but it’s easy to fix. Throw some new gas rings in and you should be good to go.
Broken locking lugs
This is what a normal, healthy bolt face looks like:
But bolts can get damaged, like this one:
Yes, this one’s a little extreme. Still, the more you shoot and as your AR-15 ages, make sure to check the locking lugs on your bolt.
Look for signs of wear along the back in the corners. Stress fractures can develop and the lugs can eventually break completely off. Not good.
The rifle may still function if only one lug breaks. I’ve seen this a few times after a long day of live fire drills when guys were cleaning their rifles. They had no idea anything was wrong until then.
Check your bolt every couple of thousand rounds for stress fractures so you can replace the bolt before something like this happens. Or go with an upgraded JP Bolt and don’t worry about it unless you shoot over 60,000 rounds.
The cam pin hole is a weak point in the bolt. It has the least amount of material to handle the stresses of firing and cycling. It is the pivot point for the bolt.
As a result, the bolt can break here. This is definitely a catastrophic failure. Your rifle will not run if that happens. When you’re checking your bolt lugs (above), also check the cam pin hole for signs of stress fractures or excessive wear. HM Defense makes a bolt designed to eliminate this problem.
Broken firing pin
It happens. With every firearm. You’ll pull the trigger and no bang. Have a spare lying around. They’re not expensive and it could keep you going in a pinch.
This is another piece that also wears with time. The front edge can shear off, the pin hole can break or the pin itself can break.
The spring will also wear with time. Any of those issues can cause a complete or occasional failure to extract casings depending on its severity.
If your AR-15 platform rifle is just used for plinking and fun, I wouldn’t stress out much about a regular maintenance schedule. Check it when you feel like it and be prepared to replace parts as they fail. If you’re a competitive or combat shooter, meaning you really can’t have something break while you’re using the rifle, you’ll want to inspect and replace your bolt carrier group parts on a schedule.
Replacing parts on a BCG is similar to changing the oil in your car. Do it too often and you’re wasting money. Don’t do it at all, and you’ll eventually screw yourself. The schedule you’ll want to follow will be based on your round count, so keep a record of how much you’ve shot.
The schedule needs to be based on how many rounds each part is rated for. For example, if your extractor spring has a minimum service life of 3500 rounds, then replace it at around 3500 rounds. Call the manufacturer that supplied your BCG so they can tell you what your parts’ minimum round life should be.
I recommend contacting the manufacturer because the ratings may not be the same across the board. There’s no point in replacing a JP bolt every 10,000 or 20,000 rounds when it’s good for 60,000.
Dig through enough forums and you’ll find general recommendations that can work fine for you. But if you really care about your rifle absolutely never failing, get the facts straight from the manufacturer. If you don’t know your manufacturer, or they don’t know how to answer your question, you should probably invest in something of better quality.
There are plenty of good AR parts manufacturers on the market right now. JP Enterprise, Spike’s Tactical, Aero Precision, Radian, Daniel Defense, Faxon, Failzero and Lantac, just to name a few. All of them make mil-spec bolts that should serve you very well.
My nickel boron bolt carrier group from Spike’s Tactical works great. I recommend staying away from the super-cheap options. I also recommend buying a complete bolt carrier group from one manufacturer. The chances of you getting a complete bolt with bad rings are far less than if you piece the BCG together from different manufacturers. Take my JP bolt and Rainier bolt carrier failure (above) as an example…first-rate parts that don’t fit together well.
Getting a BCG with higher-end coatings will improve its service life. Nickel boron, TiN nitride, black nitride finish, and other similar coatings improve wear resistance, lubricity and corrosion resistance. The bolt carrier group should run better and last longer. Which coating to get is the shooter’s preference. Functionally I don’t see enough difference in them to really care. Visually, each finish has its own distinct look.
Skeletonized, low mass or lightweight bolt carrier groups are for people who like to tinker with adjustable gas blocks, buffer weights and recoil springs to have the absolute softest, fastest-shooting AR possible. Other than that, they look cool, I guess. If that’s what you’re after, go for it.
The AR-15 or M16 bolt carrier group is the heart of your rifle, and therefore not something you should go cheap on. Certainly not bottom of the barrel cheap, anyway. Don’t get attached to yours and don’t neglect it, either. Definitely, share any stories of BCG failures that highlight any issues I didn’t address as well as your go-to BCG supplier.
Matt Sandy is an Arizona-based gunsmith who competes in both USPSA and PRC matches.
Had a Sharpes Reliabolt shear the lugs on both sides of the extractor after the first day at the range (maybe 100 rounds in). So yes, it can happen with brand new parts.
Luckily Sharpes was all over it when I emailed them. Sent me a new one and only asked I send the broken one back in the box they sent me the new one in. Said they had a bad batch of bolts that didn’t go through the hardening process properly. No problems with the new one since then (10,000 plus rounds later)
Sorry it had to happen to you, but I like seeing stories like this. I am often sceptical about claims of “through hardened” and “hard anodizing”, etc. I always assume it is just a marketing gimmick like the infamous undercoat on a car.
Anecdotes like yours assure me it matters.
That’s crazy, glad that was the extent of the malfunction.
One thing to note, JP uses a different style of gas rings that they advertise as not passing that ‘test’. I’ve heard of standing the bolt up as one means of checking the seal, but its not the end all result. Check the manufacturer instructions over at JP, its right there in the instructions.
I was wondering this myself.
In general, I have found the “stand it up on the bolt” test to be useless. The only ones that ever passed it were mil-spec carriers with mil spec bolts using 3 piece rings.
Any bolts with mcfarland rings or JP’s one piece rings don’t pass in most/any carrier.
All my QPQ/nitrided carriers don’t pass with most bolts.
The only bolt that passed in everything ran in nothing because it was out of spec. and wouldn’t unlock right most of the time.
For grins one day after trying to help a friend trouble shoot some reliability issues in his AR, I mixed and matched everything I owned for this test, and most combinations failed.
All the as-built combinations run fine.
my ragged edge gaming gun is a rubber city armory lightened carrier with a JP bolt on a stretch 16 barrel. Collapses fast and smooth utterly failing the test. Runs fine. For the googles and the sanity of people who play with such combos, the stretch 16 gas port, tube length, and carrier key gas adjustment do not play well together. Can’t open it up full bore and still fit the lock screw in fully.
Good to know. My experience is primarily with mil-spec stuff so I guess I need to look into this a little more.
that’s the first vurp video i’ve seen.
It’s never a bad idea to get a second bolt from the same manufactuer and keep it in with your rifle. And a few sets of gas rings are dirt cheap. Sons of Liberty Gun Works out of San Antonio include free gas rings for life with their guns.
“Sons of Liberty Gun Works out of San Antonio include free gas rings for life with their guns.”
Is that your company, JWT?
It is not. I don’t make parts for them, or guns, or anything. They just make a hell of an AR and are good folks to deal with.
Does anyone with a higher round count have a complete BCG they can recommend in the affordable range $69-$150?
I’ve got a few uppers I have to use the OEM BCG on and I would rather have a whole spare instead to pop in instead of worrying about replacing firing pins or extractors at the moment I’m ready to go shooting.
BCM complete carriers run around 170 or so. Spend the extra, they’re worth it if you want quality.
Wcarmory has them well in your budget.
^100x that. Toolcraft are good to go.
Bought a tool craft nickel boron 9130 steel BCG arm or ally for $111.51 shipped after some research. It looks like there is a possibility that it is actually toolcraft that makes the Bravo Company BCG for them so I appreciate the great advice from you guys as I was ignorant of the toolcraft brand.
Toolcraft does not make bolts for BCM, microbest does. Microbest are far better than toolcraft. Microbest also makes bolts for SOLGW and Sionics. If you are not looking to pony up for an LMT enhanced bcg or Sandcutter with an LMT ebolt the SOLGW bcg is the best of the mid/upper tear bcgs on the market due to the small parts they use vs BCM and Sionics.
I’m an AR beginner but I would think most would benefit from this post. There’s a chit ton of Youtube video’s with everything you need to know about AR15’s too…
There’s so much damn stuff on YouTube it’s tough to figure out what’s bullshit and what’s not.
Any particular channels that have *solid* information on AR platforms?
broken firing pin retaining spring!!
How could you forget that? Way more common than many of the items listed here.
“”firing pin retaining *pin*
I don’t doubt it, but oddly enough I’ve never experienced it. thanks for bringing it up though, definitely something to be aware if.
Toolcraft all the way, or just overspend if you like
It is nice you folks are putting out some beginners guides. Educating newbies, and less informed is a good way to boost your viewers. However, there’s improper terminology all over this arcticle. For example, the term bolt is used several times in the place of correct term of carrier several times. Things like this may cause people to still be confused on the platforms mechanics. If you clean this up, it will be a go-to arcticle for a very long time.
Kripes…I wish I could edit my own post.
Thanks for pointing that out, not sure how I let those errors slip. They should be all corrected now.
one should always carry a quick fix field kit for the rifle, that contains small parts, new bolt, firing pin, springs, ect. Items that can be stowed in a cough drop tin. parts are cheap and can be useful when you need them. When your out in the field or range, the day can be ruined by not being prepared. beats carrying a broken rifle all day due to small parts breakage.
What AR15 bolt group works best with a Bump Stock? Or fostech trigger?
any Full auto one, doesn’t matter. as long as it’s timed right with your spring, buffer weight and gas length. any one of these differ and your cyclic rate goes off. It’ll be too slow, too fast, recoil too soft or harshly, ect.
My first AR-15 is a Bushmaster from about 1994 and doesn’t have staked gas key screws. I guess I should do that one of these days.
Echoquestions.com will have videos showing you the necessary cuts (full auto) for the BCG to have in order to work with their echo trigger, but the short answer is that mostly everything is going to work.
This is for Geoff PR(in a movie theater)…Mrgunsngear and Military Arms channel are great. 704 Tactical. and an old one NZC(?). Nutnfancy but he’s looooongwinded. Lots of others! Sorry I couldn’t get this as a reply…
While this article is probably unnecessary for that majority of TTAG readers, it is nice to have a break from the politics. Reviews, technical articles, innovations, etc, are always refreshing.
Someday I’ll Upgrade My Stock BCG In My RUGER AR-556MPR To Probably A Nickel-Boron Grade From Lantac Or Equivalent, But Until Then I Have No Problems With My Current One That Came With The Rifle…
Reading about the seemingly endless/potential problems, as I understand the comments, with the M-16/AR-15 type rifle, I wonder as to why people bother, recognizing that there is no explaining personal tastes or preferences.
Once upon a time I had a Winchester made Garand Rifle, that I put literally thousands of rounds through, some of which were probable excessive handloads. Other than a broken firing pin, I never experienced a failure of any kind with that rifle, that while cleaned regularly after being fired, otherwise had no particular mechanical attention. Are the AR-15/M-16 type rifles unduly sensitive, I cannot say. Was that Winchester Garand unusually rugged, don’t know that either. I can only speak to what I personally have experienced. Comment/questions from readers welcomed, if any are interested.
FFS the AR15 (and its derivatives) is not a DGI gun! The gif even shows how the system uses an inline piston!
Lubricity and ease of cleaning for black nitride or NiB are VASTLY superior to a milspec phosphate finish. Saying otherwise is utter nonsense.
Also, has the author never heard of Rocksett or the like? High temp threadlockers do exist.
AR15 is not DGI?
Yes, the stock, mil-spec AR is indeed a gas piston design. If we’re getting technical and as accurate as possible about terms, here you go:
Now, few people realize this and the community at large realizes we need to differentiate ARs that have a simple gas tube vs. a piston on top of the barrel operating on the BCG. It’s a convenient way to split the rifle into two categories of operation, but it overshadows the actual operation of what we incorrectly call “DI” ARs.
I’ll continue to call the original design “DI” since few people know or care to dig into the actual operation of that BCG design, but it’s fun to understand how it works. It’s really an ingenious design, far better than most folks give it credit.
No comments concerning FA v SA BCGs?
I disassembled the BCG on my AR10 in order to polish the extractor hook and the ejector pin. When I reassembled it if found a spare part on my workbench: a tiny coil spring, 0.090 inch in diameter and 0.200 inch length.
I have no idea where this spring came from. I looked on your web page hoping to find mention of it. It almost looks like the Extractor Spring (G) in your bolt carrier group exploded diagram. But I don’t see where this would go.
There is the possibility this might be a spring for the retainer pin on the Takedown Pins.
Any help appreciated
Nice article. Newbie to AR platform. Very easy to understand especially with good pics and illustrations. Thanks!
It’s been said but it brought me to the comment section. JP uses a one piece gas ring set up and they will tell you that it will fail the stand up test used on 3 piece gas rings. I also wanted to add 158 carpenter steel is normally better than 9310 steel bolts. The 9310 is 8% stronger when it is heat treated properly. The problem is it is a lot harder to heat treat properly. An individual can’t test how well the treatment was performed and the manufacturer has a hard problem telling too. If they miss on the heat treatment it might fail at 500 rounds or it might fail at 1500 rounds but it will fail prematurely. Don’t gamble on your life when you can spend a little more on the 158. Microbest makes one of the best bcg right now. They make for BCM Daniel Defense Colt and Sons of Liberty gun works to name a few. All fine options for your AR.