Firearms

Bolt-Carrier Assembly: The Heart of an AR-15

Black bolt carrier assembly on a light gray background

The following is a specially adapted excerpt from “The Competitive AR15: Builders Guide” and “The Competitive AR15: Ultimate Technical Guide,” books by Glen Zediker and Zediker Publishing. For more information, visit www.ZedikerPublishing.com or call (662) 473-6107.

The AR-15’s bolt and carrier are the heart of the rifle, so knowing the ins and outs of the bolt carrier — weight, platings and coatings, firing-pin hole size and bolt choices — can make your AR run more smoothly and reliably.

In an AR-15, the bolt and carrier are the “action.” The upper receiver secures the barrel and gives the bolt carrier a place to ride. The bolt carrier houses the bolt and resets the hammer as it rides back and forth within the upper. The carrier also bears the brunt of the gas pressure during operation, via the carrier key attached to it; this pressure moves it back to cycle the firing mechanism. The bolt carrier has nothing to do with headspace; that’s all in the bolt itself. Using the same, correctly headspaced, bolt in different carriers is accepted as safe.

Bolt carriers come in two basic configurations, and then there are a few unique takes. The essential configurations differ mainly at their back ends. An AR-15 carrier has a shorter section of full diameter at its tail; an M16 carrier has a longer section. The M16 carrier has a shrouded firing pin and so requires a “large-collar” firing-pin style. The extra collar diameter is necessary for it to be reset.

Black bolt carrier assembly on a light gray background
Bolt-carrier designs are essentially defined by two formats for the back end. Specifically, how much back end it has. M16 bolt carriers (shown at back) and most aftermarket premium carriers have a longer portion of full diameter. That makes them heavier. The author prefers heavier carriers. More mass means the bolt will stay locked a little longer prior to case extraction. Among other things, that improves case condition for reuse. If it’s a carbine or you’re running max loads through a rifle, that’s also more margin against pressure-induced extraction difficulties, as well as against case failures from the same cause. Note the collar sizes on the firing pins; the bigger collar goes with the M16-style carrier, necessary due to the shrouded firing-pin recess.

To the semi-automatic owner, the differences in these carrier designs pretty much come down to weight. The M16 style is heavier, just over an ounce. That’s a good thing, I say. But I can’t tell you to run out and buy one. In my experience of buying dozens of them, the M16 carrier is effectively a gray area with some suppliers, and some treat these carriers as NFA items. Some suppliers won’t sell an M16 carrier to you without you producing a tax stamp; others will. I’ve been denied enough times to realize there is some sort of subjective standard. Best I can tell, because M16 carriers are necessary for correct full-auto function in the rifle, some manufacturers and suppliers are just really careful about selling them to the public. That’s the reason I don’t just say for folks to run out and get the M16 part — they won’t always be successful. But if they can get one, they will be happy with it.

The good news is, there are some accessory-market “match” carriers that possess forms virtually identical to the M16’s. This, as anticipated, is done to increase carrier weight. Some also like the shrouded firing pin because they think it’s more reliable. In a clean rifle there’s no difference. Dirty guns are unreliable, no matter which parts they’re made from.

Silver carrier piece with flutes and extra weight on a light brown background
Originally designed by Dan Young and now produced by Les Baer, here’s a chunk of pretty carrier metal, with features. The flutes help it hold lube and reduce surface area for easier sliding, and the extra weight suits it well for higher-pressure ammo. Plating makes for wipe-down clean-up. Some have concerns that plating makes the carrier harder than the upper receiver, but, guess what? Steel is harder than the upper receiver too. Keep it all lubricated!

When I have the choice, I like to have a heavier carrier, and also a carrier and bolt set that’s all it can be. That means an aftermarket, proprietary carrier design and select bolt. I cannot honestly tell you that a rifle shoots one bit better on target with a high-dollar carrier system, although there are some advantages to the premium parts. Otherwise, “good” is good enough, as long as good is not a conjecture. “Good” means genuine mil-spec. A bolt carrier set from a major maker will usually function just fine.

Coated or plated carriers are an asset, but not an advantage. Lemmesplain. The accessory finish makes them clean up easier and is slicker than Parkerizing or oxide. Oil, of course, is slicker than any finish, and if either is lubricated like it should be, then there’s little to no point in reality for this notion. Mostly, they clean up easier. Again, they don’t perform better, meaning that your rifle won’t shoot even one bit better with a plated or coated carrier.

I am not a fan of lightweight carriers, although exist on the aftermarket. The idea is to soften the recoil pulses, which are also disruptions to aiming point location, and the reality is keeping the bolt locked longer has the same effect. There have been attempts to improve the fit of the carrier to the upper to, ostensibly, improve accuracy, and this has always been problematic, and proven pointless in gains. There actually has to be some play so that the alignment of bolt and barrel lugs can be correct. AR-15s are different.

Bolts? When I can find them, I use Colt’s since I’ve yet to see a bad one. Machining is nice, especially on the faces, and firing-pin hole sizes are consistently smallish. Firing-pin hole size should always be checked and critiqued.

Blueprints call for a 0.058-inch-diameter firing-pin hole, which is ideal. If the hole is too large, then pressure-induced primer problems will, not can, surface. Excessive pin-hole diameter leads to primer structure failures, meaning they pierce. It’s not the fit between the pin and the hole in the bolt, just the size of the hole itself. A bolt with a hole that is too big will likely not allow the use of (normally accepted) maximum-pressure loads. How big is too big? I say 0.062 inches is too big. Measure with a caliper, which is not precisely accurate, although it shows whether the hole is in the smaller or larger range.

Black carrier with 0.058-inch-diameter firing-pin hole on gray background
Blueprints call for a 0.058-inch-diameter firing-pin hole. That’s ideal. Mostly, we’re guarding against a firing-pin hole that’s too large. To that end, 1/16 (0.0625) or #53 (0.0595) drill bits can be used as checks. If the first fits the pin hole, find another bolt. If the #53 won’t go, use that bolt with confidence.
The author recently bought a package of six firing-pin retainers, and three were usable. The ends didn’t match in alignment or lengths, and no amount of finesse could get the bad ones installed. After proving it’s a worthy part (by installing and reinstalling), the author polishes and dresses the tips on the retainer to make sure it keeps working. The original-style solid retainers (right) are the best; considering frequency of use, such a purchase is not extravagance.
Here’s what can happen (will happen) if the firing-pin hole is too large and the round is near max pressure.

Back to the bolt carrier. Look over a new one for any rough spots, ridges, or burrs and don’t be afraid to smooth them over. Plating won’t budge, although the phosphate-finished steel will. Then keep the darn thing lubed! I use a little grease on the underside and then oil all over. Same with the cam pin. Grease on its top side and oil elsewhere. Keep all this assembly clean.

Pay particular attention to the recess where the back of the bolt lives. It’s tough to get the carbon deposit out of there, which means most folks don’t. The grunge build-up there can be a prime cause of the mystery malfunctions that tend to turn up after, say, 2500 rounds of neglect. Likewise, keep the bolt tail (the area behind the gas rings) free of deposits. Carbon can be tough stuff. If you get tired of scrubbing with brushes, try GM Top Engine Cleaner. Strong stuff. Get it at your Chevy dealer’s parts counter.

Carrier screws need to be staked.
Normally, the carrier key is installed on a purchased carrier. If not, installation is not a simple matter of screwing it down. Note the stake marks to retain the screws (that’s why the screw heads are knurled). Carrier screws need to be staked! The author often sees this ignored. The key gets hammered continually, and if the screws aren’t adequately retained, they will loosen. Not may. A loose key means malfunctions.

I’ve encountered a rash of poorly-made firing-pin retainers, and a couple that had been just wrecked to ruin during factory assembly. I mean that these could be removed but not reinserted. The culprit was poor alignment and execution on the retainer leg tips. It’s very difficult to remedy malformations of this nature, short of part replacement. This is a major issue because of how often this pin must be removed for maintenance. I have started using the old-style solid firing pin retainers and had zero problems.

Last, and this will be made a part of the focus of another article, is the carrier key. For now, make sure the screws are staked! Glue isn’t enough, yet that seems to be the trend anymore in factory installations. I’ve seen quite a few that weren’t staked.

Two black carrier keys on a white background
The author recently bought a package of six firing-pin retainers, and three were usable. The ends didn’t match in alignment or lengths, and no amount of finesse could get the bad ones installed. After proving it’s a worthy part (by installing and reinstalling), the author polishes and dresses the tips on the retainer to make sure it keeps working. The original-style solid retainers (right) are the best; considering frequency of use, such a purchase is not extravagance.

Have you ever had problems with your bolt-carrier assembly? What did you do about it? How will you put these instructions to use in making sure your gun is in tip-top working condition? Share in the comments section.

[gzediker]

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (27)

  1. I recently compared a bolt carrier from a colt to a TNW AR15. I was shocked to see that the screws on the colt were not staked at all. The TNW was only staked on one side. I told the shop owner and he didn’t seem concerned. I think that concerned me the most. I told him that he should contact Colt and try to send it back to them. What are your thoughts on that. You State that Colt is all you use?
    Thanks Gary

  2. Can someone tell me why the bolt carrier is so long for? many other semi auto rifles has a smallar group, so why not this one?

  3. Sorry for the delay in responding. Was out. I agree that an M16 carrier should not be a restricted item. There’s nothing it can do, by itself, that violates my understanding of NFA regs. However! In my experience, the 16 carrier is effectively a gray area with some suppliers, regardless of whether or not it’s “legal.” Some won’t sell one to you without producing a stamp, some will. I’ve been denied purchases plenty enough times to realize there is sort of a subjective standard. The 16 carrier is necessary for proper full auto function. I’m just careful in suggesting anyone go out and purchase any unique 16 parts. I think some manufacturers and suppliers are likewise careful in selling anything for 16s also. What was miscaptioned?

  4. I wish this article had covered gas rings in detail. I understand that you want good and tight rings so the bolt doesn’t move freely inside the carrier, but I don’t know why that’s important. What exactly will fail if you have loose rings?

  5. The author definitely should have done a better job than to imply that an M16 BCG is restricted by NFA laws. The fire control group, sure, but not the BCG…And should have just stuck to the characteristics of the two different types of BCG’s. While the AR-style platform can be a very accurate rifle, accuracy shouldn’t be the main goal. ALL semi-automatic weapons have a certain amount of slop built into them; it’s the nature of the mechanism in order to reduce malfunctions. Granted, some have tighter tolerances than others, but the main consideration with AR’s is to keep them reasonably clean, i.e. not letting carbon build up. There were some good points in this article, but the proofreading and captioning make it hard to take it seriously.

  6. I’ve got a brand new XM15 that fails to load on nearly every round. After trying multiple mags and rounds I finally swapped the BCG with my friends M&P15 BCG and it works flawlessly. Trying to get Bushmaster to warranty replace my BCG but I may just buy aftermarket. Had several people tell me the problem is mags, ammo, barrel, even lubricant. The BCG should be the first suspect, not the last.
    http://s344.photobucket.com/user/Johnny34PDX/media/HPIM9745.jpg.html

  7. If the solid pins are better why did the Army tell us to throw them away and use the stamped design?

    1. The stamped pins are MUCH cheaper than the milled pins…not a big deal for you and me but a big deal if you’re paying for a hundred thousand or so. Also, the solid pins do not tend to stay in the bolt well and fall out during disassembly (I’ve lost two in recent months when I wasn’t paying attention which swapping bolts) which was a big problem during the initial deployment as soldiers kept losing them, the cheaper pins stay put until you pull them out which is actually a desirable feature.

  8. Thank you!

    I am 100% for what the NRA “stands” for. The “way” they go about it? Not so much. Both sides of the Second Amendment fence seem to think that scare tactics are the way to win this fight. My personal opinion rests with leaving well enough alone. If there is anything that needs to change, I feel that would be enforcing the laws on the books and holding those that do not more accountable.

  9. There is so much wrong with this article, from the obvious misinformation regarding so called “machine gun” parts, to the miscaptioned photos, that it is difficult what to know is accurate info. After all, if I can clearly see that there are at least a few facts that are wrong, then how can I possibly trust anything the author has written?

  10. Amen! Ooh wait I cant say that anymore huh. I cant believe the this is going down. when did we start having to worry about our google searches or if I make another AR will I be put on a list. Everyone so none shelant about these new laws taking rights away by the time someone reads this I might locked up in a reeducation camp for an indeterminate amount of time. half of the people reading these websites need to get there head out there ass because more they watch every click on the internet from everyone not just list people how do you think you get on a list , every text email phone call, when did we turn into Russia if you stand up to the government and an there shady big business laws you get picked up and held with out a trial for as long as they feel necessary WTF they are having national guard running drills through our streets blackhawks hummers buying billions in ammo FYI Not the ARMY, Homeland Security they are planning to strip our rights or take us to war doing because they now they are one more backdoor agenda law away from a civil war

  11. Excellent article for me, as I am new to AR’s. I have had a lot of problems with the firing pin retainers and usually have to order a supply as often they can’t go back in and I have to use a new one. My DPMS .308 Panther came with the solid pin and I’ve ordered replacements for my 5.56 AR’s. I also really appreciate the advise on the real benefits of coatings. I would like to have seen more pictures of what he was describing.

  12. Been on the range a lot, and on the armorers position. A properly maintained M16/AR15 should be able to fire several hundreds of rounds in a single day if necessary between cleanings without a weapon related malfunction. As a private, all my NCO’s tried to throw the blame on me when the weapon wouldn’t cycle correctly straight out of the arms room, and I found out later in my career that the armorer hadn’t done HIS level of maintenance on the weapon. I could have gotten it white glove clean, and I would have had problems. When you have gas tubes improperly installed, gas keys poorly staked or not staked at all, and barrels improperly mounted, no amount of cleaning will make the rifle perform as it should. When I started making some changes in my arms room, our M16’s started seeing a significant reduction in the malfunctions. Not to say you shouldn’t clean your weapon, but it doesn’t have to be white glove clean to function properly.

  13. Built an ar-15 with the goal of going as cheap as possible. Upon completion, found that it would always FTE or FTF. Upgraded gas block, only slight improvement (now cycled 5.56 once in a while, but never .223) Tried different buffer weights, springs, magazines, nothing seemed to work. Finally tried my friend’s BCG and all of a sudden all problems went away. Put my bcg in his gun and the problems followed it. I bought my BCG for $100 from DSArms so I called them up to complain and before I could even say “you suck” they offered to replace it at no charge and cover the shipping. Got my new BCG and fired it the other day and after about 100 rounds of all types of ammo, including crap Tuula ammo, I had not one jam. Not even 1. It was the bolt carrier group the whole time!

  14. A decent read for the novice but a bit of misinformation concerning the legality of M16 bolt carriers…anyone can own & use a M16 carrier without violating the NFA. Yes, typically machinegun parts are controlled items but there are some exceptions to the rule as there is some commonality of machinegun parts and “regular” parts but if by going what the author is saying as gospel then even using the M16 firing pin would be forbidden. To further this argument, then why do companies such as Colt use M16 carriers in their civilian 6920 carbine…is colt violating the NFA or making felons out of their customers? BTW Colt isn’t the only manufacturer that is currently using M16 carriers.

    1. The only parts that will get you in real trouble with the ATF are in the lower receiver.

  15. The author states “M16 parts without Class III validation are a violation of the NFA. That’s federal law.”

    This is false

    From the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE), Chief of the Firearms Technology Branch
    “…M16 bolt carriers are not designed and intended solely and exclusively for use in converting a weapon into a machinegun and are not any combination of parts from which a machinegun can be assembled. Further, an M16 bolt carrier is not a firearm as defined in the GCA or a machinegun as defined in the NFA. 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.”

  16. A lot of good info here, but I would like to point out a couple of things.

    I think plated BCGs have slightly more advantages than suggested in this article. Chrome, NiB, and TiN coatings are used on gun parts and metal cutting tools to reduce wear and increase lubricity. Both of those things lead to increased service life, especially in less than ideal situations (i.e. Inadequate lube, high temperatures, or dirty environments). For a properly maintained rifle, it probably makes no noticeable difference though.

    According to the ATF letter linked below, M16 BCGs are ok in semi-auto ARs. M16 fire control parts, disconnector, hammer, etc may be another story though.

    http://www.gandrtactical.com/images/archive/ATF%20M16%20Letter.pdf

  17. Please note…mere use and possesion of an M-16 carrier in and of itself IS NOT considered a Class III weapon by the BATF and is perfectly legal to wn and use. Many websites that sell M16 carriers have copies of the BATF determination on this matter available for reading/download on their website, or you can always go to the BATF site itself and confirm this. But the carrier itself IS NOT considered a Class III weapon in and of itself, but merely a firearm part.

  18. Nearly 15 years as an armorer (USAR), I have seen most every failure to function because of poor lubrication and generally poorly maintained M16s.You are ‘preachin’ to the choir, Bro. It took many years to teach the importance of proper operator maintenance, and in many cases, I may have had better results teaching proper cleaning & lubrication to a pile of rocks.

    1. Staked means that the metal around the head of the bolt is struck by a pointed item (like a nail setter) to deform and push the metal against the screw, thus keeping it from vibrating loose.

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