Tag Archives: over gassed

AR15 Gas System Enhancements, Part 2


Reducing the influence of excessive AR15 gas system pressure is most directly done reducing the pressure itself. Here’s how!

Glen Zediker

This is the second of two articles on ways to tame down an “over-functioning” AR15 gas system. Aside from running more reliably, reducing the evil influence of an overly-rapidly unlocking system improves cartridge case condition, which means longer case life. The first article talked about ways to increase the time the bolt stays locked, or delay its unlocking, however you want to see it.

Going more directly to the “source,” there are also ways to reduce the actual amount of gas that gets to the bolt carrier key and that’s up now for this one.

Adjustable gas block
Here’s an adjustable gas manifold. It’s a way to restrict the flow of gases through the system. Don’t get too greedy! Make sure to err on the side of function. These are probably the single most effective means to tame over-function. Check this one out HERE.

An adjustable gas manifold or “gas block” is an effective means to restrict the amount of gas that gets into the system. This device attaches at the port location, replacing the existing manifold (or front sight base if it’s a standard-configuration build) and will have some manner of valving function whereby propellant gases allowed to pass through the gas port in the barrel, through the manifold, and into and through the gas tube are restricted. Some incorporate a valve that regulates the passage dimension. Others provide a vent, more or less, to expel excess gas. I prefer the “valve-type” over the “bleed-off-style” devices.

Installation is straightforward, and these are available from a wide array of sources, so it shouldn’t be hard to find one that will fit even a custom-profile barrel. Standard for this area is 0.750-inches diameter. What matters is that the inside diameter of the manifold matches the outside diameter of the barrel at the connecting position.

There are different approaches to using this device but it’s really pretty simple. Figure out the minimum gas flow necessary to function the action and then open the flow-control screw adjustment a half turn more to give a little safety margin. Don’t get greedy. I shut one down all the way (minimum flow) and then open it up until the rifle functions.

The only foible on an adjustable manifold is that it has to fit in with the architecture of the setup you have. A retro-fit requires removing any muzzle device that might be installed and, of course, removing and later reinstalling the gas tube (make sure you check that it isn’t binding).

I have used other products that provide alternate means to do the same thing, like a gas tube with a valved adjustment mechanism. Sometimes something like that is best for anyone wanting to run a more standard gas manifold system. They work just fine, and dandy.

adjustable gas tube
There are other means for softening the system, and this adjustable gas tube is an example. Others include the “pig-tail” gas tubes that spiral around the barrel to increase tube length/volume. They all work…

Other gas tube modifications that work have been those formed in a spiral that wraps around the barrel, and I’ve seen tubes with expansion chambers (area of larger volume) along the span of the tube. What’s happening with these isn’t reducing the amount of gas, it’s just giving it more distance or room to weaken its presence.

The best solution I’ve yet encountered is fairly new and is an adjustable bolt carrier key. This requires no modification or labor about the barrel, and also works with virtually any AR15. Remove the old carrier key and replace it with the adjustable key.

adjustable carrier key
Here’s fairly new: an adjustable bolt carrier key from Sun Devil. David Beatty hit a long ball with this device, the ADIGS. I like it because it can be added to virtually any AR15 out there, even one that needs to maintain outwardly stock appearance. Works great. See more HERE.

A good while back I talked about gas port pressure and propellant burning rates and cautioned against using a propellant on the slower-burning side of “suitable propellant chart” center. To reiterate, I don’t think any propellant slower than Hodgdon 4895 should be used, but I know full well I can safely extend that range one more step to say something like Varget or RE15 is the limit. Slower propellants create more gas port pressure because they peak farther down the barrel, nearer the gas port location. Related: I recommend to anyone who’s going to do a longer custom barrel to request that the builder relocate the gas port another inch forward. There’s more gas contained in a longer barrel for a longer time: more pressure hits the carrier key as a result.

long gas tube
It’s common for an NRA High Power Rifle to get its gas port relocated forward another inch, or even two. The reason is because the 24-inch+ barrels we run “trap” more gas inside, which increases the pressure available at the gas port. The port farther forward gives more time and room (all the gas goes out when the bullet exits the muzzle).

No doubt, if you load up an AR15 with a heavy carrier and related parts then combine that with a gas restriction device, the range of propellants can move one or more steps slower-burning. In any of my full-blown across-the-course race guns, I can construct and successfully deploy loads that would wreck a rack-grade AR15. Don’t mess with that. Enjoy smoother and “softer” function and the assurance that you can run closer to a maximum load without fear of the odd and inevitable “pressure spike” causing problems. That’s why to do it.

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

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 ZedikerPublishing.com