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, please check out 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.
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.
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.
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.
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.
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.
About the author: Glen Zediker has worked professionally with some of the greatest shooters on the planet, as well as leading industry rifle builders, manufacturers, and proven authorities on gunsmithing, barrelmaking, parts design and manufacture, and handloading. And he does pretty well on his own: Glen is a card-carrying NRA High Master and earned that classification in NRA High Power Rifle using an AR-15 Service Rifle.
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