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Why Subsonic 300 Blackout Needs a Pistol Length Gas System

Posted by Nic Johnson on

Why Subsonic 300 Blackout Rounds Need a Pistol Length Gas System.

When entering the subsonic world you are going to be factoring in a suppressor, different ammunition types, barrel sizes, and gas systems, buffer tube weights. All of which can have an effect on how well your rifle performs.

One of the most common questions we get in the shop is why do you need a pistol length gas system when running sub-sonic rounds through a suppressed 300 Blackout AR15?

Because of the rounds ballistic capabilities of moving below the speed of sound, there is an inherent problem with the amount of powder load used to propel the round. Too much powder and the bullet will break the threshold of sound, defeating the cartridges purpose. When entering subsonic world you are going to be factoring in a suppressor, ammunition types, barrel sizes, gas systems, buffer tube weights. All of which can have an effect on how well your rifle performs.

Perhaps the most important aspect of subsonic rifle performance is the gas system. Although they vary in length by manufacturer, the typical lengths are standard and measured from the receiver to the gas port as follows:


AR-15 Gas System LengthsPistol Length – 4 inches
Carbine Length – 7 inches
Mid Length – 9 inches
Rifle Length – 12 inches

The length of the “gas port” is an integral part of firing a subsonic round since lower amounts of powder are used. The more gas pressure that can be directed towards moving the bolt backwards, the more consistent the platform.

In essence it’s all about creating enough back pressure. Here’s a simple animation that shows how an impingement gas system works:

AR-15 Gas Impingement System

  1. Hammer activates the firing pin, setting off the round.
  2. The gas from the powder propels the bullet down the bore.
  3. Gas is redirected backwards though the impingement system and assists the action of the bolt.
  4. The bolt engages with the next round in the magazine and sets the round into battery. (Ready to fire)

Since a subsonic 300 blackout round is heavy and uses less powder than supersonic rounds, it’s crucial to utilize as much gas as possible to cycle the firearm. This is obtained by positioning the “gas port” close to the receiver of the firearm with a pistol length system, thus creating more back pressure during the time the bullet is traveling down the barrel (also known as “dwell)” and pushes the bolt back and chambering a new round.

In a nutshell, placing the gas port closer to the lower receiver allows more time for gas build up while the round travels down the bore.

Another factor to consider is the weight of the buffer. The heavier the buffer weight the more back pressure is needed, and the opposite for a lighter buffer weight. The weight will vary depending on your choice of barrel length, and gas system. Do your research and make sure to ask a gunsmith which will work best. Many times when a SBR is short stroking and not cycling correctly as the buffer tube spring or weight may not be tuned correctly.

Typical buffer weights are categorized much the same as barrel and gas system lengths. Carbine Buffers weights are typically 3 oz. and Rifle H3 buffer weights can often can get close to 6 oz. Mid sized buffer weights H1 or H2 can be found somewhere in between and adjusted if necessary. For a subsonic rifle running 300 Blackout it can be expected that the buffer weight will be on the lighter side and the spring coil with be shorter than a typical gas system. These factors will vary greatly between the needs of your platform, as each system you employ will result in a different need or adjustment. It’s important to keep this in mind when tuning your subsonic rifle to run at top performance.

If you are thinking of purchasing a 300 Blackout rifle and plan to shoot subsonic rounds, remember that there are a few things to check off your list before hitting the range: A proper gas system, tuned buffer weight and spring, and polished feed ramp.

We hope this short article touch on the basics help you on your way!

All of our ammo is batch tested, hand inspected and backed by our guarantee. If you are having trouble running our subs though your rifle and would like expert advise, you can contact us at or give our shop a call at (218) 547-3015 and we will help you diagnose the problem.

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  • Great article
    Buildinga 16 in 300 carbine and have been looking for this exact information on gas port distance
    Would have built my 8.5 pistol with the shorter gas platform
    But did not know at the time it mattered
    Used the carbine 7 on length, it cycles fine just over subsonic but soon as I go under it will not cycle
    Even with a suppressor.

    And o really don t want to cut the spring because the lower is also used on my 8.5 223
    I may. Cut it when I get a dedicated lower but that will be after this carbine build

    Bruce Strong on
  • Great article, answered some hard to find answers to many questions. And all in one spot! Thanks!

    Dennis Jackson on

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