Adjustable AR Gas Guns, My Take ~ VIDEO


Ft Collins, CO –-( “O Human Race, born to fly upward, wherefore at so little wind dost thou fall?” ~ Dante

Gas Guns: At our popular DTI Urban Rifle Courses, the question of “gas adjustment” is ever on the minds of students.

In these times, most of us use semi-automatic, military rifles. There are many choices.

The Stoner System (AKA: “Direct Gas Impingement,” or “Pressurized Receiver”), brings high-pressure gas, via a gas-tube, directly into the receiver, momentarily pressurizing it, and thus driving the normal cycle of operation.

Most M4 rifles currently being manufactured employ the Stoner System, and are not “gas-adjustable.”

However, several manufacturers are now producing after-market gas blocks and accessories which allow gas-adjustment, even on Stoner-System rifles, because some shooters insist on it.

Most other military rifles employ a gas piston, connected to an op-rod, and most of these are gas-adjustable, from the factory.

Gas Block Adjustment

“Gas adjustment” is incorporated into a control knob on the rifle which will allow the user to change the percentage of gas (diverted from the interior of the barrel as the bullet goes past the gas-tap hole) which is subsequently used to drive the action, versus the percentage that is deemed “excessive” and is thus vented to the outside.

When a rifle, gas-piston or Stoner System, is “under-gassed,” the main symptom will be sluggish operation. In the extreme, under-gassed rifles will “short-cycle,” meaning that they are no longer capable of normal semi-automatic functioning.

With gas-adjustable rifles that are shot-cycling, increasing the percentage of gas that drives the system (while simultaneously decreasing the portion that is vented off) will restore normal functioning.

Often, rifles that function normally when clean and well-lubricated become sluggish as they get dry and dirty. Again, increasing the gas percentage that drives the system will keep the rifle going until it can be cleaned and re-lubricated.

Most semi-automatic, military rifles that are not gas-adjustable (which includes most M4s) are deliberately “over-gassed,” so even when dry and dirty, although they do become slightly sluggish, it is not so profound that they begin to short-cycle. They are designed so that there is always plenty of gas to ensure normal functioning, despite unlubricated moving parts and excessive gas leakage.

So, is there a down-side to too much gas?

Yes, several!

Superlative Arms .625" Adjustable Gas Block
Superlative Arms .625″ Adjustable Gas Block

Over-gassed rifles will generate more felt recoil than rifles that are correctly gassed. Probably a moot point with those chambered for 5.56×45, but sometimes a significant issue with those chambered for 7.62×51.

Over-gassed rifles will experience more wear-and-tear/parts-breakage than correctly-gassed rifles.

Most significantly, over-gassed rifles will often have bolt movement so violent that the magazine cannot push-up the next round fast enough to be fed into the chamber usually. The result is that the cycle of operation yields a chamber full of thin air, or with the bolt “over-riding” the next cartridge and stopping with lugs dug into the middle of the top of the brass case.

Of course, bolt velocity can be adjusted via heaver or lighter recoil springs as well as via gas-adjustment. In fact, with non-gas-adjustable M4s, changing recoil springs/buffers is about the only way to address the preceding symptoms.

The point of all this is that a reliable, serious rifle, one you want with you during exciting times, is neither over, nor under-gassed. Via gas-adjustment, or via correct selection of the recoil spring, it runs reliably through all kinds of circumstances, with different brands of ammunition, and despite continuous lack of maintenance.

Many “recreational” rifles fail this test and will fail you at a very inconvenient moment.

Serious rifles only need apply. And, when your rifle is gas-adjustable, you need to know how and when to adjust it correctly, to keep it within a generous “Goldilocks Zone.


Defense Training International, Inc

About John Farnam & Defense Training International, Inc
As a defensive weapons and tactics instructor John Farnam will urge you, based on your own beliefs, to make up your mind in advance as to what you would do when faced with an imminent lethal threat. You should, of course, also decide what preparations you should make in advance if any. Defense Training International wants to make sure that their students fully understand the physical, legal, psychological, and societal consequences of their actions or in-actions.

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I don’t bother assembling uppers for my family that do not have adjustable blocks (SLR my first preference, followed by Superlative). We run suppressors oft-times and having an adjustable block just makes shooting suppressed more reliable, less dirty, and just nicer. Other uppers assemblies I’ve put together that have benefited from adjustable blocks include: • DPMS pattern 308 was WAY, WAY over-gassed. No amount of springs and buffers would improve it. Adjustable block took care of that problem. • 6.5 Creedmoor: Same issue as the 308, a bit less so. Problem solved • 7.62×39 uppers two over-gassed and one so… Read more »


My AR10 in 6.5 Creedmoor benefited greatly from an adjustable gas block and my .300 blk pistol can be adjusted to where it will shoot supersonic and subsonic loads without change by use of this item. Good stuff.