AR Gas System Lengths

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In today’s post the discussion turns to AR gas system lengths. If you have ever looked at the specs of an AR-15 you will see they always list the gas system length on direct gas impingement rifles. I will explain what this means in the following order:

  • Direct Impingement Gas System
  • Carbine Length Gas System
  • Mid Length Gas System
  • Rifle Length Gas System

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Direct Impingement Gas System

First of all let me explain what exactly a direct impingement gas system is. As you fire an AR-15 some of the gas vents off through a small port and covered with a gas block. The gas moves down a steel gas tube going into the bolt and pushing it backwards ejecting the spent case and loading the next round. So essentially they use some of the gas from each round fired to cycle the weapon to eject and load the next round.

AR Gas System Lengths: Carbine Length

Now the first of the gas systems is a carbine length gas system. This is the shortest of the all the discussed gas systems. It is used on shorter barreled weapons typically anywhere from 10”-16” barrels. It has the shortest gas tube and is known to have a snappier recoil especially on longer barrels due to the fact the dwell time, the time the bullet takes from passing the gas port to moving out of the barrel, is much longer causing more gas to be pushed back through the bolt increasing felt recoil. This extra gas can cause more wear on internal parts as well which can lead to reliability issues.

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AR Gas System Lengths: Mid Length

The next system is the mid length gas system. It was designed to help fix some of the associated problems with the aforementioned carbine length gas system. The mid length gas system is has a longer gas tube and is made primarily for barrel lengths of 14.5”-18”. The benefits of a mid length gas system are that it mitigates the issues with shorter gas systems on longer barrels by increasing the dwell time to help make the forces imposed by the gases lighter and being more efficient.

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AR Gas System Lengths: Rifle Length

The last system to discuss is the rifle length systems. This set up has the longest gas tube and farthest port from the receiver. It is made primarily for barrels 18” and longer. It will have softer recoil and if used on a 16” barrel may cause the bolt to short stroke where it does not get enough gas to cycle the weapon. This reason makes it most useful on the longer barrel lengths used on AR platforms.

AR Gas System Lengths: Conclusion

Some people automatically want an answer to which is the best AR gas system. Following the criterion primarily of barrels lengths mentioned about will help you pick the best AR gas system for your situation. And by best I mean which will give you optimal performance and give the least amount of wear while keeping the weapon functional. Hopefully this simple breakdown will simplify the way these three common gas systems lengths are set up and for what purpose. Thanks for reading and check back for more content.

Garrett W.

  • A rifle length gas system can be effectively used on a 16″ barrel if the gas port size is adjusted accordingly. Oftentimes you will hear of short stroking issues because the barrel manufacturer failed to make this design change.