Adjustable Gas Blocks: A Field Guide
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With so many barrel and ammunition combinations in the AR-15 world, the platform’s unique and ordinarily reliable gas operation system could run into trouble in some circumstances. An easy fix for this pitfall can be an adjustable gas block, enabling the user to fine-tune their rifle’s operation. This article is part of our ongoing Guide to Everything AR.
In This Article
Adjustable Gas Block Comparison
Adjustable Gas Block Reviews
1. PSA Click Switch
If you’ve ever shopped for AR components you’ve more than likely encountered South Carolina’s Palmetto State Armory. Their Click Switch Gas Block makes it easy to tune your gas pressure with a front-facing click adjustment screw that clicks between adjustments to prevent unwanted movement and slippage.
The black nitride ensures it keeps you black rifle, uh, black and it comes with PSA’s 100% Full Lifetime Warranty. Made in the good ‘ol U.S.A.
2. JP Enterprises
Minnesota-based JP Enterprises is more than a one-trick pony when it comes to adjustable gas systems. They offer at least eight different series of blocks to include low-profile blocks, Pic rail blocks, and — probably most importantly — one that replaces the standard A2/M4 front sight.
The latter block, the JPGS-2FS, is a no-brainer for those using military profile rifles and are looking to add some recoil management via a variable gas manifold.
3. ODIN Works
ODIN Works sells an adjustable gas block that they argue isn’t just tunable– like other blocks– but is truly adjustable across 20 different clocks of detent. Rather than being side-adjustable, it is front adjustable, and they include a long Allen key to pull that off.
Constructed of nitrided steel with a stainless adjustment screw and spring, they are marketed for 0.750-inch diameter barrels. The cost is about $90.
4. Seekins Low Profile Gas Block
Seekins Precision’s Low-Profile Adjustable Gas Block is a bargain for what it offers, typically running about $60. Available in three diameters– 0.625, 0.750, and 0.875– they are made of steel and are no-frills but deliver, nonetheless.
For a step up, Seekins also markets a Select Adjustable Gas Block for about $120 that uses an audible and tactile throw lever switch for a whopping 30-setting adjustment setting that is just about impossible to back out.
Best yet, it can be adjusted on the fly without tools and marked for “sweet spots.”
5. Superlative Bleed Off Gas Block
SupArms could very well be the Jedi masters of adjustable gas blocks for direct impingement AR-15s. Forward venting with a 30-position range, their blocks channel blowback gas away from the operator rather than just restricting the flow back into the bolt carrier as other adjustable blocks do. They sell a full range of these blocks for various sizes to include 0.625, 0.750, 0.875, and 0.936.
These blocks, since the bleed-off port allows the excessive pressure to be exhausted out of the block reducing blow back, are a dream for use with cans. They also make them in stainless models with a matte finish in addition to the standard Melonite finish for those looking for a little more flash.
6. SLR Sentry Gas Block
SLR Rifleworks markets several blocks including their Sentry 7 adjustable gas block. Mounted with a choice of either set screws or clamps, the Melonited finished 4140 steel block has 15-position adjustability.
For a cost of $125, they deliver a lot for the money. For about $70 more you can get a Sentry 9 in Titanium.
7. Wilson Combat Adjustable Gas Block
Tweaked with a front adjustable flathead-screwdriver slot with 16 settings — more than twice the range of some other blocks — Wilson Combat’s newly introduced adjustable gas systems are impressive. They come pre-installed with a matching gas tube and in a myriad of lengths including Rifle, Carbine, Mid-Length, Intermediate, and Pistol.
As for the block, it’s of nitrided 4140 steel construction and uses set screws to attach. Did we mention that it has an auger system in the adjustment screw to cut carbon fouling? This means that if you get gummed up after a few thousand rounds but are away from the cleaning bench, just work that screw in and out of its range to clear the plug.
AR Gas System 101
Eugene Stoner, the father of the AR-10 rifle in the 1950s and by extension its AR-15 little brother in the 1960s, designed the gun to run on what is known as a direct gas impingement system.
In the most basic breakdown, this system bleeds off hot gas from the barrel created by burnt propellant from a fired cartridge and uses a small amount of gas to power the action — slamming the bolt back into the buffer, ejecting the spent case from the breech, and loading another round into the chamber.
This gas, traveling upwards through a port in the barrel, feeds into a gas block, which is fundamentally a chimney attached to the barrel. There, the gas is pushed down the path of least resistance through the gas tube back into the bolt carrier which cycles the action.
If there are hiccups in this process, it is likely due to the gun either getting excess gas too quickly or not enough gas to operate. Both situations can be addressed with an adjustable gas block.
Signs of an over-gassed AR
When an AR-10/15 is receiving too much gas from the gas port/block/tube into the gas key of the bolt carrier, it is referred to as being “over-gassed.” To some extent, a lot of ARs and uppers on the market today are intentionally designed and timed with large gas ports and blocks, along with the longest gas tubes possible for the length of the rifle, purposely over-gassing the guns to a degree.
The reason for this is to enable ordinarily “military spec” ARs to reliably feed and cycle chunky, cheap, and often problematic Eastern European ammunition with lacquered steel cases rather than quality brass. Going past this, over-gassing can help a gun perform under harsh conditions such as when dirty with carbon build-up after heavy use or when cycling wet or funky ammo.
The rule of thumb is that, in the race between under-gassed or over-gassed, more is better and can be more easily corrected. However, some guns can be significantly over-gassed to the extent it causes failures or makes the gun considerably less reliable.
Some of the most common tell-tale signs of a badly over-gassed AR system are:
- Harder felt recoil.
- Misfeeds in which the fresh cartridge is chambered before the spent case is fully ejected. This double feed, the dreaded Type III malfunction, can be one of the hardest jams to fix.
- Bolt-over-base malfunctions where the bolt is catching the topmost round about an inch in front of the base because it is outrunning the magazine.
- Empty brass ejected too fast. This will throw brass forward and to the far right of the port. The optimal ejection is to about 3 o’clock.
- Accuracy issues.
- Rapidly worn parts on the bolt carrier group, especially on extractors, bolt teeth, and keys.
- Damaged or prematurely worn buffers due to excessive recoil.
Signs of an under-gassed AR
On the other side of the coin from an AR that is receiving too much gas through the action, under-gassed guns can produce far more problems as the gun simply isn’t going to cycle ammunition properly, typically because of short stroking.
- Spent cases fails to eject or, if they do, just fall lazily out of the chamber, potentially causing a Type II malfunction.
- Hot brass ejects towards the user’s face, backward into the 5- or 6 o’clock angle position.
- Fresh rounds fail to feed into the chamber or aren’t picked up from the mag.
- Bolt won’t lock back on an empty magazine.
- Potential problems with accuracy.
Why would you need an adjustable gas block?
First off, not all ARs need an adjustable gas block installed. Far from it. Most decent rifle and carbine builds, using a fixed gas block that is properly aligned over a proper-sized port on the barrel, will work fine with most factory ammo. Just about every law enforcement duty/patrol rifle and military contract M16/M4 build uses a fixed gas block.
Adjustable blocks are there to cover the margins, step up durability, and provide a measure of insurance when using more exotic ammo. Also, they can help a shooter get up on target faster for return shots as they lighten felt recoil.
Moreover, they are key to helping an AR run reliably with a suppressor or when using a lighter or specialized low-mass Gucci BCG, both of which can change how the platform cycles.
Finally, if you live in an area with extreme temperature changes, an adjustable block can help improve reliability. At lower temperatures the cartridge propellant can burn at a sightly slower rate, creating a difference in volume at the gas port and affecting dwell time.
Opening a block’s adjustable gas setting a bit more can solve this issue and keep the gas flowing more freely, allowing you to use the same gun and ammo on both 100-degree days in August and 20-degree days in March.
To be sure, there are a number of other fixes for over-gassed guns such installing a heavier H2 buffer and red spring or SSS spring, swapping out a standard BCG for a full-auto M16 carrier or other, heavier, BCG. The same can be said for under-gassed guns, where a buffer kit with a lighter buffer or BCG can be installed.
Drilling out the gas port is an option for some more capable gunsmiths (don’t drill your gas port, please). The problem with this is it’s often an over-correction. A pendulum spring from one nightmare to the other. Further, changes in ammo or suppressing the gun could quickly see a return to the same problems.
However, the best case can be made here for threading the needle between the two issues by simply installing an adjustable gas block. These blocks, typically tunable at the user-level under field conditions, can offer a ready solution to reliability issues. Further, they aren’t that expensive, hard to install, or difficult to use.
Few direct gas impingement AR-15 platforms come with an adjustable gas block installed from the factory. Nonetheless, swapping out a fixed block for an aftermarket AGB isn’t that hard. Be sure to follow the manufacturer’s guidelines for this and keep in mind.
Pinned on blocks are “old school” but should you go that route know that there will be some despair involved– installing the gas block roll pin is probably the most frustrating part of any AR-15 build.
Clamp-on blocks, by far, are the easiest to install but can suffer from durability issues, especially if knocked or misaligned. However, they secure to the barrel with substantially more contact area than setscrew type blocks and are better for barrel harmonics.
Those that attach with brass set screws are a safe bet and are commonly encountered. If you run the set screw type, you seriously need to investigate the use of rockset/red Loctite and dimpling the barrel during the installation to add peace of mind if the barrel isn’t pre-dimpled.
Pro-tip: Before selecting a gas block– either fixed or adjustable– be sure that the handguard or rail system you are running will accommodate it. Further, if you do go adjustable, be sure the handguards allow access to the adjustment and jam screws. Remember to check your adjustable screws often, as they tend to walk out after as little as 300 rounds or so due to the nature of barrel vibration and repeated gas spikes, a factor that Loctite can help alleviate.
As there are a few different types of adjustable gas blocks, always be sure to follow the manufacturer’s guidelines on adjustments. Going past that, initial testing will involve the adjustment screw backed off just one revolution from being fully tightened or closed. At that setting, load and fire a single round of the ammunition type and manufacture that you typically plan to use and check for function.
Should the rifle bolt fail to lock back completely on the empty magazine after firing or show other signs of under-gassing, open the screw about a half-revolution and repeat the test until you have the proper function, then adjust it about another quarter turn and tighten it down before a final test.
Keep in mind that as few as four turns will open the block all the way on some blocks, creating a de facto fixed block, while fully closing the adjustment will shut it down. The adjustment will typically be by an Allen wrench (or two), which can be stored in a baggy (to prevent rattling around) inside a compartmentalized grip such as a Magpul MOE.
Better blocks will have a toolless adjustment, and we’ll get to that.
When it comes to construction, try to avoid aluminum blocks. Remember, the heat and gas coming off the gas port can wear a pipsqueak block out very rapidly. Steel, preferably 4140 or harder, preferably nitrided, is recommended. Exotic blocks made from high-stress materials like Inconel and titanium cost a bit more, but odds are you will never wear them out.
- Army Study Guide, Malfunctions, Probable Causes, Corrective Actions (ArmyStudyGuide.com)
- Firearms New, Gus Norcross, How To Install AR Gas Blocks
- 80 Percent Arms, AR-15 Buffer Weights: What The Difference Is, and How to Choose, October 13th 2020
More on accessories:
March 16, 2023 — Our adjustable gas block recommendations remain unchanged after a thorough review of this guide. We’ve updated images and links.
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