Posted by Nathan Schueth on Aug 23rd 2016

​Frequently Asked Questions: Which Gas System Length is Best for My AR?

We are often asked about the gas system length is best for a specific barrel length. The answer to that question is combination of personal preference and technical requirements, but we are happy to assist with information that may help one pick what is best for them.


Does it Matter for Function?

Generally speaking, no. Faxon custom tested all their gas port diameters to ensure function with the given gas system length. If we offer it, it’s only after we are 100% confident that the gas port diameter is the ideal size for the barrel and gas system length.

However, going to a longer gas system length can reduce wear and tear on your weapon system. We’ll explain more in detail below.

So Why Different Lengths?

It’s common to see various gas system lengths on the intermediate length barrels, commonly 16” and 18”, which are offered in carbine/mid and mid/rifle, respectively. Those various lengths are offered due to customer demand or the system length is dictated by a specific profile.

For example, the 18” .308 Medium Taper barrels are offered in both mid and rifle as some customers prefer a length over another for cosmetic reasons. The 18” rifle gas block is 12” from the front of the receiver, so customers wanting to use a shorter handguard may want the shorter gas system to ensure the gas block is covered.

For profiles, 16” barrels are offered in both carbine and mid length. Most 16” barrels are mid-length, but the “M4” profile is carbine length to match what the US Military offers in their standard profile. For other barrels we manufacture as in-house developed profiles, we typically offer them in mid-length only.

How Does a Longer Length Help with Long-Term Life?

The gas system length determines the amount of dwell (time) from the ignition of the primer and the amount of dwell from the bullet passing the gas port to the bullet “uncorking” which allows pressure to drop quickly. All gas system lengths are a balance between caliber and barrel length. Small, high-pressure rounds like 5.56 allow for long gas systems. Larger, low-pressure calibers like 300 BLK prefer shorter gas systems.

For a firearm, it’s best to have the longest time from ignition to the unlocking cycle of the rotating bolt. The longer the dwell from ignition to unlock, the lower the pressure on the operating components including the pressure to operate the action and the pressure on the bolt to the extension. While components are manufactured to handle high pressures, the lower the pressure, the longer the life of the component.

So What Does Faxon Recommend?

As a general rule, we recommend the longest gas system we offer for the given caliber and barrel length. However, any barrel that we offer is tuned to the given caliber and length, so shoot and enjoy!

Thanks to AIM Surplus for the illustrative photos. Check them out for the full line of Faxon products and exclusive AIM configurations!