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

By Glen Zediker published on in Firearms

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]

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Comments (21)

  • Glen Zediker

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    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?

    Reply

  • David Fricks

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    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?

    Reply

  • Coffeeguy

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    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.

    Reply

  • PDX

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    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

    Reply

  • Icorps 1970

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

    Reply

    • Gregory Markle

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      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.

      Reply

  • Michael

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    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.

    Reply

  • John Black

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    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?

    Reply

  • Freedom Fighter

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    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

    Reply

  • Frogger

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    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.

    Reply

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