Choosing An AR-15 Gas Block thumbnail image

Choosing An AR-15 Gas Block

5D Tactical - 13th Dec 2022

The gas block is pivotal to the function of the AR-15. Besides the specific size of the gas port in the barrel itself, choosing a quality part is also important to getting the seal and set correct. As a part that's critical for proper function, it's surprising how affordable most gas blocks are. 

What is a Gas Block?

The AR-15 Gas Block is a small metal device that clamps onto the barrel at the gas port and connects to the front of a gas tube as well that comes out of the upper receiver. There are some pretty expensive brand name gas blocks and there are also cheaper ones out there available. Typically, you'd want to avoid the ones that are too cheap for fear of quality and reliability issues - however, more often than not we see individual gas blocks being sold on sale for very low prices. 

What Does a Gas Block Do?

The gas block acts as a means to redirect the gas from the barrel’s gas port to the gas tube. AR15 gas blocks need to be able to handle the pressures that entails - up to 68,500 PSI give or take at most (though it is usually less). That’s an enormous amount of pressure to redirect which is why they have to be set in place on the barrel with a lot of clamping force. 

When a round is fired through the barrel, its role is to direct some of the gas from the port into the gas tube. The gas is then used to push back the bolt carrier group which is what creates cyclical action in direct impingement type AR15's.

Standard or Adjustable Gas Blocks

Standard gas blocks are very commonly available. These will typically come with AR15's that are factory built or if you pick one up off the walls at your local gun store. Standard gas blocks only allow a fixed rate of gas and pressure from the gas port to the gas tube. They tend to be very strong and reliable in that regard — it’s hard to get more bombproof than solid metal. However, one of the draw backs of using standard or fixed gas block is that if you have an over-gassing issue you won't be able fine tune that and will be forced to remediate the situation with a specific buffer weight and spring combination to reduce the perceived felt recoil. 

Benefits of Adjustable Gas BlocksAdjustable AR15 gas block

Adjustable gas blocks allow you to adjust the size of the channel connecting the gas port and gas tube to increase or decrease the flow rate and pressure of the gas as it feeds back into the gas tube. Having the ability to fine tune your gas system can help reduce overgassing, especially if you're running a suppressed build. With the combination of an adjustable gas block, heavier buffer weight and spring you can easily create an extremely flat shooting system. 

The one we carry in particular is a fairly simple system and extremely reliable. All you need is a long allen key to close off the gas port (righty tighty) or if you want to open it up for more gas flow then you turn to the right (lefty loosey). There are slight, but distinct notches you'll feel when turning - similar to when adjusting an optic so make sure you count how many clicks you make in case you need to readjust or start over when tuning at the range.

Some may quote parts reliability or the fear of parts failure which is why there are people who swear off adjustable gas blocks and only used the standard ones. We disagree because in our opinion, you use this to 'set it and forget it.' Perhaps over time the set screw may get caked in carbon and erode or it will be forever set in that position but for it to "fail" and stop doing what gas blocks do even if that were to happen... unlikely. That's why we have adjustable gas block is just about every company and personal build.  

Gas Block Tuning

The AR-15 is all about energy. We want to harness energy from the expanding gas of the cartridge through the gas port, block and tube to send the bolt back to extract the spent cartridge. We then want to capture enough of that energy into the buffer spring to then send the bolt back with enough force to feed a new cartridge into the chamber. It needs to do all of this reliably with a wide range of environmental and ammunition variables, otherwise it won’t be trustworthy.

When we tune at the gas block, we’re tuning the entrance variable. The variable of how much energy we send through the rest of the “algorithm” is the BCG’s mass, the buffer, the buffer spring and the overall lubricity of the system. It can help the builder diagnose the system while he/she is tuning the algorithm with different weights of buffers, BCGs and spring weights. This will also heavily depend on the shooter’s ammunition preference.

Gas Block Size and Different Barrels

Gas blocks are measured by their interior diameter, generally measuring .750”, .625”, .875” .936”. If unsure what the diameter of the barrel is, a builder could use a set of calipers to find out quickly or check the manufacturer’s website.

.750 is generally standard size for most barrels. This is where the prices will offer the best value given these gas blocks are manufactured at a much greater scale. It’s also the easiest size to find quality adjustable gas blocks.

.625 is used for pencil or thin barrels. While giving up rigidity and mass for holding heat, these barrels are the lightest available. It’s common to pair these with a lightweight gas block. This does reduce the reliability and chance for failure, but weight at the front of the rifle is the enemy of lightweight builds.

.875 and .936 /43 the heavy or bull barrel standard diameters. Used for maximum rigidity and heat absorption, these barrels are the chunk. Very heavy to use, but well regarded for their accuracy.

Low Profile Gas Block

BCM low profile fixed gas block

low profile gas block is almost always a fixed gas block without adjustable capability. AR-15 handguards can sometimes leave very little room for material to even add attachments. These gas blocks are called 'low profile' because they're able to fit essentially any for handguard or rail size. However, these are considered an industry standard. Without these smaller gas blocks, builders can’t use the slimmer handguards if paired with a larger, perhaps "fancier" gas block though this is rarely ever a problem. 

Get Your Next Gas Block from 5D Tactical!

AR-15 Gas Blocks are one of the most important parts of the system and poor quality parts will sabotage your next build. Knowing it’s function, size format and that quality matters will set a builder up for success. Whether a builder wants a hyper lightweight .625” gas block, maximum rigidity bull .936” gas block or something in between, 5D Tactical wants to be a part of it!