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Adjustable Gas Block: AR-15 Edition

By now, each component of the AR-15 has gone through a series of revisions, iterations, and offshoots that hone in certain aspects of the performance of the firearm. By evoking the concept of a firearm performing well, we must also ask “at what?” If the difference between a gas-piston system and a direct impingement AR-15 were like comparing vehicle drive train options, then choosing an adjustable gas block would be deciding the merits of one's suspension. Some perform well in rough terrain, others are easy to maintain or replace, and none of it matters if the equipment is not installed correctly.

The A2 Front Sight Post Gas Block has been around for a long time.
The A2 Front Sight Post has been around for a long time, but is not adjustable.

Before the popularization of suppressors, fixed gas blocks were much more common. A traditional baffle suppressor increases the internal pressure of an AR-15 enough to cause problems for the system as a whole. Dwell time refers to the amount of time that expended gas is able to vent through the gas port of the barrel. The closer the port is to the muzzle, the less time it has to send gas into the upper receiver, pushing back the bolt carrier. Attaching a suppressor impacts the system like adding more length to the barrel, increasing the dwell time, albeit less than if the barrel were to be extended.

Even though we're dealing in microseconds, more dwell time means more gas, more gas translates into an increased velocity of the bolt carrier group as it travels into the buffer tube, increasing the felt recoil and parts wear. In addition to this, failures to extract or feed and increased fouling can be symptoms of an overly gasses system. An adjustable gas block can mitigate this if set properly but brings its own challenges to the table.

AR 15 Gas Block instal
Getting rid of the M4-style gas block with its integrated sight tower and replacing it with a low-profile gas block allows the use of a slim, free-float handguard that improves handling and accuracy.

Not all adjustable gas blocs are created equal. In addition to the quality of the overall manufacturing, each one has its own features and quirks.

Method of Adjustment

At the macro scale, we have two types: one with dedicated positions, and another that adjusts along a spectrum. Adjustable gas blocks with fixed positions are more popular on firearms similar to the AR-15, especially those with gas-piston operating systems. The most common settings are regular/open and suppressed/restricted, with some featuring a third adverse setting for harsher conditions.

Colt CRP20SS Adjustable Gas Block

Gas blocks that adjust along a spectrum forgo specific settings and allow the shooter to tune their adjustable gas block specifically to their guns. Ideal for set-it-and-forget-it builds that will either never see a suppressor, or be suppressed all the time, the advantages of a more precise setting require a little forethought especially as longer rails that ride past the gas block adjustment point remain the standard. Beyond this, they are more accommodating across barrel lengths and gas port sizes.

superlative arms titanium adjustable gas block
Superlative Arms Titanium Adjustable Gas Block

Some companies, like Superlative Arms have branched out into new territory with their bleed-off system. When engaged, it vents some of the pressure above what is needed to cycle the firearm away from the shooter. This serves a double purpose of sending less irritating gas into the shooter's face as well as mitigating some of the issues that plague adjustable blocks.

Adjustable Gas Blocks that use a screw to restrict the gas flow will face both erosion and carbon build-up over time. For some, this could be a common maintenance item, and for others, simply a consideration, depending on how much one shoots. If we were to measure the number of rounds fired a year across the population of all gun owners, the Pareto distribution would certainly come into play.

Method of Attachement

Just slide it on and tighten it down, right? Sort-Of. Some gas blocks are pinned in place, and others clamp-on, while a third common choice uses a set screw to hold the block in place when driven into a dimple on the barrel.

Of these three options, pinning is the most secure, but carries with it the most catastrophic failures when the system breaks apart. These problems are rare, and pinned Gas Blocks are more common on upper receivers that ship complete from the manufacturer. They are near-permanent when finished with a weld.

An adjustable gas block held in place by a set screw commonly finds home on .223/5.56 home-built or modified AR-15's. Many barrel companies ship their product with a set-screw dimple already in place on the barrel, but a competent gunsmith can generally facilitate this if needed. Since the set-screw will face heat, pressure, and vibration, some form of threadlocker is highly advised.

seekins precision gas block
Seekins Precision Gas Block. Note the Set Screw on the Bottom.

Seekins Precision Low Profile Adjustable Gas Block
MSRP
: $60
URL: seekinsprecision.com

Clamp-on options require nothing from the barrel except that it is properly sized. Depending on the shape of the clamp, some rails might not fit over the adjustable gas block. Plan accordingly, use threadlocker.

Getting the Right Setting

There are multiple practices for what an adjustable gas block should be set at. Since this doesn't apply to gas blocks with pre-determined settings, we're only referring to ones that use a screw or similar device to adjust how much pressure is vented through this portion of the system.

JP enterprises adjustable gas block
JP Enterprises Adjustable Gas Block

JP Enterprises Adjustable Gas System
Weight
: 2.37 ounces
MSRP: $120
URL: jprifles.com

Variables to consider:

  • Will the gun be used suppressed, unsuppressed, or both?
  • How many different types of ammunition will be fired through it?
    • If multiple loads will be used, which ones produce the most and least amount of pressure?

Springs lose their stored kinetic energy over time: they wear out. So if you're expecting to replace a buffer spring soon, kill two birds with one stone by re-adjusting your gas block at this time as well. A good checkup on the health of your firearm does not go to waste.

A properly adjusted gas system consistently ejects spent casings in a predictable direction, and cycles the bolt back far enough to achieve last-round bolt-lock reliably

If multiple grain weights of projectiles are to be used, begin with the weakest load to ensure the firearm will cycle.

Begin unsuppressed, opening the gas block to its maximum. From there, begin tightening it down by consistent increments (1/4 turns) until the firearm will not reliably cycle. Back off a quarter turn at a time until the firearm consistently cycles and locks back, especially in three-to-five round strings. Pay attention to where the brass ejects from the firearm: close to 90 degrees out of the ejection port is a decent average.

Add your suppressor and evaluate its performance. Is the brass pattern consistent? If the firearm is throwing the spent casing almost directly forward, it is over-gassed and should be tuned down.

Remove the suppressor and confirm that the firearm will still cycle and lock the bolt to the rear.

Gas Block Nuances

The weight-conscious have lighter options from some manufacturers. Even though we're dealing in ounces or less, as they reside closer to the muzzle, they can feel like pounds over time. Titanium gas blocks, such as those by Superlative Arms, Master of Arms, or the SLR Sentry 7 often weigh in less than an ounce overall.

superlative arms titanium adjustable gas block
Superlative Arms Titanium Adjustable Gas Block.

Superlative Arms Titanium Adjustable Gas Block
Diameter Options
: .625, .750, .875, .936
Weight: 1.08 ounces
MSRP: $210
URLsuparms.com

A poorly fit gas block will leak gasses either from the gas tube or from where it meets the barrel. If not set properly over the hole, the firearm may not even cycle. Both problems are mitigated by a witness mark.

An AR Gas Block will come in one of four sizes, measured by the interior diameter in inches: .625, .750, .875, and .936. These sizings must correlate with the diameter of the barrel at the point where the gas block mounts. The majority of barrels are cut to .750 at this point, but the thinner pencil barrels often come in .625. Barrels at .875 and .936 are typically found on heavier versions barrels chambered in 6.5 Creedmoor or .308 Winchester, not to mention other larger projectiles.

govna adjustable gas block
The MicroMOA Govna Adjustable Gas Block made a splash when it first appeared, then vanished.

Archetypes

Whether to create the flattest shooting, most reliable, or lightest weight AR-15, it can be tuned to the needs and desires of the user through part selection. An adjustable gas block adds adaptability to every build, be it tuning for minimal recoil impulse, or the ability to push through hard use in an austere environment. Whether shooting suppressed, or in pursuit of the ideal competition AR-15, an adjustable gas block punches above it's weight in tuning the AR-15.


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