AR-15 Gas Systems Breakdown

As some of the editorial staff of Blue Collar Prepping delve deeper into the AR-15 world, questions have come up regarding the more esoteric and non-standard options available on the AR platform. This week, we'll discuss gas systems.

The AR platform of rifles is (with few exceptions) a direct impingement rifle with a gas port near the muzzle. In layman's terms, this means that a small hole in the barrel near the muzzle diverts some of the gasses from the burning powder back to the bolt to cycle the action. This hole is contained by the gas block, and is almost always located under the front sight.

There are four gas systems available on an AR-15 upper. These systems are defined by the distance between the upper receiver and the gas port and are also rated for specific barrel lengths.

• Pistol gas systems have a 4" port distance, and are ususally only on barrels under 10".
• Carbine gas systems have a 7" port distance, and are rated for 10-18" barrels. These are the most common gas system seen.
• Midlength gas systems have a 9" port distance, and a 14-20" barrel rating.
• Rifle Length gas systems have a 12" port distance, and are for barrels in excess of 20".

Needs will determine barrel length, so rifle and pistol gas systems are pretty easy to select. Carbine and midlength systems have a fair amount of overlap, though, which leads to a few considerations that must be made to determine the optimal gas setup. In general, a midlength gas system will have smoother, lighter recoil, but may have problems cycling light loads. The carbine system is more reliable, but is louder, snappier, and a bit less effective with a suppressor.

Different gas systems require different buffers as well.  If you're going to swap uppers around, be sure that that you have the appropriate buffer installed in your gun.

At left, a rifle buffer.
On the right is a carbine buffer.

With the proper combination of barrel length, buffer size, and gas system, your rifle will operate reliably and accurately. Next week, we'll discuss the pros and cons of some non-traditional calibers.

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