The Use Of Adjustable Gas Blocks To Tame Hot AR-15 Gas Systems

BY Herschel Smith
3 weeks, 3 days ago

This guy doesn’t like the idea.

I do not at all think that an adjustable gas block is the best answer to tame an over-active AR-platform gas system—and that means one that operates too quickly, too forcefully. When that’s at issue, gun parts and cartridge cases get beaten unnecessarily, and, if it’s “bad enough,” function problems result—most commonly ejection and feeding failures. Various barrel length, gas system length and cartridge combinations conspire to create this condition. It’s most common with shorter systems, such as carbines, and also with longer than usual barrels using a rifle-length system.

An adjustable gas block houses valving apparatus that’s user-adjustable. Some vent away or bleed off unwanted gas, others restrict or limit the flow of the gas into and through the system. Those that vent tend to hold up better longer.

Clearly, the idea is to install the adjustable gas block and then turn it “down.” Great idea! It’s easy, effective and it doesn’t last forever.

The valving in the block succumbs to the same heat- and pressure-induced erosion (flame cutting) as does a standard system (and as does the chamber throat and gas port) and will change, and it will get dirty, maybe corroded, and might lose its capacity for adjustment. Gas-induced erosion, by the way, increases space volume and that will effectively reduce pressure, so that means a tuned gun might not work after time unless the valve can be readjusted.

Adjustable gas blocks are best in the hands of competitive shooters, and are most wisely used only in rifle-length gas systems. One can work well to tune out percussion-induced rifle movement and will improve spent case condition. The farther ahead the block is from the rifle chamber, the less intense is the pressure and flame.

A recent article I wrote here discussed the importance and effectiveness of making essential “architectural” changes in the gas system to get closer to the original Stoner rifle spec combination of port location and barrel length (which works just fine, and dandy). However, I will quickly concede that valving might be the only reasonable solution. If we have to work within already-set specs that aren’t nearly ideal, then we may also have to work around that, and there’s the value of the valve.

[ … ]

I turn one all the way down, all the way closed, and then open it up until the gun works, plus a little more to ensure full function. The “full-function” test, by the way, and this is important, is that the bolt carrier assembly will go to lock-back on the last round. It takes a little more carrier velocity to get it back far enough to trip and set the bolt stop than it does only to strip the next round from the magazine. Don’t cut it too close!

I have to say I’m a bit unmoved by his dislike of this particular use of adjustable gas blocks.  Look, the original barrel length in the Stoner rifle is still out there and can be purchased, but it’s much less frequent to see a full assembly for sale.  Mostly, if you want one, you’ll need to order the barrel and replace the one you have, including the gas system.

So we’re dealing with typically shorter barrels now, some as long as 18″, some much shorter (14.5″ with a welded and staked flash hider, or even shorter for AR pistols).  To me, if your gun begins to malfunction and it’s obviously not the barrel or chamber, the first place to look is the BCG, the second is the buffer spring, and the third is the gas system (including the gas block).

Replacement of all gas blocks will be necessary given the second law of thermodynamics.  If gunsmiths or gun mechanics disagree with my assessment, I’m not married to it.  Please disagree, and state the reasons for your disagreement.


Comments

  1. On November 20, 2019 at 1:27 pm, MTHead said:

    Where I found the most gas problems was in 308, carbine length gas systems.
    I found that the gas returned back so fast that it tried to extract the case while it was still under internal case pressure. You can see this on the case where the ejection plunger will smug across the case head. As the bolt tries to unlock while still pressurized internally.
    When this happens it will sometimes take away just enough energy that the bolt isn’t able to move far enough rearward to pick up another round. (short stroke). It can be very frustrating. Especially if you handload.
    To fix this, adjustable gas blocks are the quickest, easiest way. Nothing else worked. As stated, one round at a time till the bolt locks back. Plus a little extra.
    Or just get an upper with a mid-length, or rifle-length gas system.

  2. On November 20, 2019 at 6:20 pm, Georgiaboy61 said:

    The rationale of using a gas block is sound-enough. The AR-platform operating system, which we can call modified direct-gas impingement, is somewhat notorious for being over- or under-gassed depending on the intended use of the platform, precise setup and other variables. Users who employ suppressors benefit greatly from an adjustable gas block, since the attachment of a “can” often causes the carbine, SBR or rifle to be over-gassed.

    Upon firing a suppressed shot, too much gas pressure can cause blow-back of the hot gases and fumes into the operator’s face via the charging handle, which is why special “gas buster” sealed charging handles are often called for. Absent this modification, a simple adjustment to the gas valve often fixes the problem.

    Many users of suppressed AR’s within the military and tactical community have transitioned to piston-based AR’s – such as the Heckler & Koch 416 carbine – as a means of dealing with these concerns. A short-stroke piston system such as that used by the HK runs cooler and cleaner than a straight DGI carbine, especially with suppressed. And since most short-stroke piston systems are self-compensating, the need for an adjustable gas valve is eliminated or at least greatly reduced.

    Users of normal unsuppressed AR’s still benefit from an adjustable gas system when their rifle/carbine/SBR gets fouled, hot and dirty from extended use. Cranking open the gas port just a bit will buy the user a bit of addition time of user before the system must be shut down to cool, be cleaned and so forth.

    The final use of an adjustable gas port is to accommodate different makes, styles and types of ammunition. An AR which is set-up to run optimally with one type of ammunition may not work as well with another, and being able to adjust the gas system can sometimes alleviate such problems.

  3. On November 20, 2019 at 6:33 pm, Georgiaboy61 said:

    To add a final point: The late Eugene Stoner’s subsequent designs after the Armalite AR10 and AR15 seem to show that he was thinking about the pros-and-cons of DGI operating systems.

    After the AR15 rights were sold to Colt, Stoner found himself in the peculiar position of competing against his own design in the marketplace. He could not design a platform too-similar to the AR15/M16 lest he find himself being sued for patent infringement – or having to pay royalties to Colt, the owners of the design.

    Stoner’s answer was the Armalite AR18, which has been called by some firearms historians as the “greatest ‘failure’ in modern firearms history.” The design, innovative in a number of ways, never caught on commercially, and when Armalite found it could not sell enough of them, the rights were sold to various concerns in the U.K. (Sterling) and Japan (Howa), amongst others. The AR18 did end up being very influential in firearms design/engineering circles, as its features and short-stroke, rotating bolt operating system proved to be quite well-regarded and widely copied.

    The take-away is that Stoner used not DGI – direct-gas-impingement – but a more-conventional short-stroke operating system, in the AR18. He also employed the proven and well-tested rotating bolt, perhaps in recognition of the fact that primary extraction had been something of a weakness in the AR15.

    Among the many present-day firms which use the AR18-style operating system, are Heckler & Koch, the makers of the HK416.

  4. On November 21, 2019 at 1:05 pm, Badger said:

    Possibly among competitive shooters and handloaders, or those that are working with some suppressed setups, the AGB has some merits. I wonder (but do not know) if a large number of folks feel the need to jump right to an AGB as a solution because they’ve been conditioned to expect pretty piles of brass on the square range at their 4 o’clock, or are chasing some notion of “softer shooting.” I read recently that there are now only 3 buffers in the regular army.mil inventory: rifle, the H2, and a hybrid for use by rifle-length gas with telescoping stock. (Source is a gent that is continually downrange maintaining the A-Z of weapons in use.)

    As long as function allows the case sufficient time to chill-out prior to extraction I’m just as fine with it (the now-worthless case) being enthusiastically sent away to my 2:30 or 3:00 as I was happy with my Garand doing the same thing. Not saying AGB’s have no use; just seems as if they are the solution du jour when some simpler methods could be explored -if- there is an actual problem. It’s worth remembering also that both buffer & spring are part of a system and should be considered together.

  5. On November 22, 2019 at 8:35 am, Name (required) said:

    The FAL has always had an adjustable gas valve. It never gives trouble. Not sure why it would be a bad idea on an AR.

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This article is filed under the category(s) AR-15s and was published November 19th, 2019 by Herschel Smith.

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