Reloaders Corner: AR15 Gas System Enhancements, Part 1


AR15s can have problems “over-functioning.” There are two essential ways to make your AR15 behave better! Here’s the first…

Glen Zediker

Right. I know this column is about handloading and reloading, so why am I spending space talking about gas system function? Well, it’s ammunition-related, or, at the least it is influenced by ammunition, and therefore also influences ammunition choices.

First, an AR15 gas system “over-functions” when it fills up too quickly and with too much burned propellant gas. The AR15 uses a “direct impingement” gas system, sometimes called an impulse system, and that means there’s a port hole in the barrel that lets gas out and through a gas tube, and this gas goes directly into the bolt carrier key and sends the whole works backwards. There’s no piston (although piston systems exist that can be fitted to these firearms) or other regulating device beyond gas port hole location and size.

AR15 bolt carriers
An M-16-profile carrier (back photo) weighs about 1 ounce more. Big big difference in slowing down initial movement. Get the right firing pin! If it’s a full-diameter carrier it will have a shrouded firing pin recess and takes a “large-collar” pin for correct operation.

The effect or upshot of over-function is overly quick bolt unlocking. The symptoms include extraction problems, damaged case rims (related), overly-blown cartridge case shoulders, excessive case head expansion, and, generally, accelerated wear on the action hisseff. As with many things, the severity of the excess function likewise increases excess in its manifestations.

What happens is that the case is swelled up under pressure inside the chamber, as it should be, but then it’s still swelled up when the bolt opens and the extractor takes a yank on the case rim to get it out of the chamber.

With respect to handloading ammunition, keeping the bolt in battery a tick longer makes a world of difference in spent case condition. The case has a tick more time to return to closer to normal dimensions and shrink away from the chamber walls. And time is, again, what this is really about. The case will be less stressed and dimensionally nearer original specs, and that means there’s “less” sizing done for next use, in effect. Case life improves and also does longer-term quality for reuse.

So. If we can delay bolt unlocking we’re seriously on to something. The simplest way to slow something down is make it heavier. Heavier things don’t accelerate as fast, they have a greater “moment of inertia,” less resistant to initial movement. Increasing bolt carrier mass is very effective. Keep in mind that what unlocks the bolt isn’t bolt movement, it’s bolt carrier movement. The bolt movement is a natural oucome to rearward travel of the carrier. Minor point but, well, there it is. I run “M-16 style” bolt carriers in all my AR15s. That’s a carrier with a full round section at the end rather than the notched out profile of the standard semi-auto carrier. And, no, an M-16 carrier won’t make a gun full-auto, and, as a matter of fact, carriers with the full-round profile are routinely encountered as “match” bolt carriers. Heavier is better!

Anything contacting the bolt carrier can increase in weight also and be effective. That effectively increases the load against the bolt carrier, and that requires more time to overcome and create movement. The buffer, for instance. I always run heavy buffers in my short guns, and also my hot-rod rifles for Across The Course use. The carbine-length stocks use a shorter spring and also a shorter buffer, and that means a lighter buffer.

AR15 buffers
Here’s an array of buffer components designed to slow the initial back-travel in the carrier. Anything helps, and more than one add-in makes an amazing difference in AR15 manners.

More about the spring’s role in all this next time, along with other more major modifications that will downright tame an AR15. And I’ll also run down a step-by-step on ensuring reliable function in a slowed-down AR15.

The preceding was adapted from Glen’s newest book, Top-Grade Ammo, available here at Midsouth. For more information on this book, and others, plus articles and information for download, visit


6 thoughts on “Reloaders Corner: AR15 Gas System Enhancements, Part 1”

  1. Purchased an ar15 pistol three years ago. Had significant problems with jamming. Platform had an M16 bolt carrier and a heavy buffer. After playing around with many ammo types without satisfaction, I installed and adjustable gas block.
    Closed the port and opened it four turns. Bolt,did not lock open.
    Opened 1 more turn and it locked. Tried four different loads with success. Opened block 1/2 more turn to assure function.
    Loaded 30 round mags with different loads and then intermixed loads with perfect function. Issue solved.
    Though different carriers and buffers may solve some malfunctions, the real problem is gas pressure. The adjustable gas block solves this problem with one part and can be tailored to the individual rifle. An adjustable gas block can reduce recoil and minimize wear on the action. If I built an AR it would have an adjustable gas block from the beginning.

  2. If one adds mass to the bolt to slow it down, wouldn’t that increases the closing force going forward, maybe causing other problems?

    1. That’s why this problem requires fine tuning. A well tuned rifle is a straight shooting rifle, where you’ve optimized and balanced gas output as a function of inertial dampening in the carrier/buffer system on the ejection stroke and back on the return to battery stroke. It isn’t just one thing, it’s multiple things and a good shooter and builder should factor those issues in.

      Just dumping components into a rifle and hoping that first they work, and second that they work well is tantamount to throwing bones on the ground and reading your future. It’s a crapshoot. Why go through those headaches when people are told all the time, measure twice, cut once or in this case build once.

      1. Just thinking out loud here… Could one not reduce the reciprocating mass and/or felt recoil by using a lighter carrier in conjunction with an adjustable gas block?

      2. What you said is completely true: it requires fine tuning. The heavier carriers do, also, create some of their own problems. There’s a higher risk of a “slam-fire” (floating firing pin tapping the primer too hard) and also bolt stop breakage, extra mass to stop. Overall, I favor the increased weight approach over the lighter-weight approach because of the, well, healthy loads I shoot. Hope this isn’t just confusing the whole thing, but my approach is a combination of carrier weight increase, gas decrease, and the actual final tuning is done with cutting coils off the buffer spring. My guns shoot soft, the cases don’t get hurt, and they only work with my handloads. It’s specialized. That’s why I tend to recommend the heavier carrier to start with. That alone usually solves a majority of over-function symptoms, but there is a lot more to it…

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