Last week on Ballistic Theory we talked about choosing the correct 300BLK load for your intended use. This week we are going to expand on that point and talk about how your gun should be built to shoot your chosen load. To quickly recap from last week, 300BLK is a specialized cartridge with loads built for specific reasons. Going on to mirror that; depending on whether your intended use is supersonic, subsonic, or flexibility between the two your gun should be built a specific way.
If you are only going to shoot supersonic only, my suggestion is just save yourself the money and buy a 7.62x39 AR. There are plenty of great builds on the market that feed from AK magazines. This keeps you from having to buy proprietary AR mags for 7.62x39 that can cause numerous headaches. However if you are adamant about your gun being chambered in 300BLK and you are only going to shoot supersonic loads, get a gun with a 16” barrel, 1:8” Twist, and carbine length gas system. You can run a standard carbine buffer and spring with that setup no problem. If you wish to suppress that setup, it will work fine the way it is. You can fine tune that setup with either an adjustable gas block, adjustable bolt carrier group, or your buffer weight.
If you are only shooting suppressed with subsonic loads, we highly recommend keeping your gun as short as possible. This allows room to put a suppressor on it so that you have the full benefits of that specific load. You can SBR the gun if you want to, however with the invention of really great pistol braces there is no need to. We would not recommend going over an 11” barrel for this application and please use a 1:7” or faster Twist if you are going to shoot 220gr as that is required to properly stabilize the projectile. Make sure your barrel is Pistol Gas length because subsonic ammo does not have the pressure to cycle properly with any other gas length. We recommend fine tuning this setup with just an adjustable gas block. You’ll only need to run a standard carbine buffer and buffer spring with this setup. That will allow you to keep the gas tuned relatively low because there is less mass to move. In return the gun will be very smooth shooting.
If you are looking to be flexible between super and subsonic loads please understand there are tradeoffs to be made. You will need to set your gun up the same way as if you were only going to shoot subsonic loads. As long as you ensure your gun functions with the weakest ammo you will be shooting, you can then just load a magazine of the supersonic rounds and the gun will cycle. The tradeoff being made is the gun will be a bit overgassed and run a bit harsh. We would still recommend a shorter barrel in this setup no longer than 11”, however that is not to say you could not run a 16” barrel. Just understand the limitations of that barrel and gas system as described above.
This article is meant to be a quick overview of the main points to look at when building/buying a 300BLK gun. It is no means a comprehensive guide to every little quirk you could possibly encounter. As always if you have any questions please feel free to reach out to us anytime.