Some say the gas impingement operating system “poops where it eats”. But is that really fair? Here are Richard Mann's 6 factual observations in the AR-15 direct impingement vs. gas piston debate.
As someone who tests and reviews guns for a variety of firearms periodicals, I’ve had the opportunity to test versions of both the gas impingement and piston-driven ARs. Here are my factual discoveries:
1. Piston-driven guns run much cleaner. Fire a 30-round magazine through a piston-driven AR and it will look just as clean afterwards as it did before you fired it.
2. Piston-driven guns run much cooler. You’ll have to shoot about 100 rounds through a piston gun and a gas gun to really feel the difference, but it is there.
3. On average, piston-driven guns are less accurate. This does not mean piston-driven ARs are inaccurate, but, looking over my test records, the most accurate ARs I’ve tested have been those that work with the gas impingement system.
4. On average, piston-driven guns cost more. This observation must be qualified with “it depends.” There are some very expensive gas impingement ARs and some piston-driven ARs that are not all that expensive. However, if you want to purchase the least expensive AR possible, it will be a gas impingement gun.
5. If you intend to run a suppressor on your AR, it has been my experience that the gas impingement guns are more suppressor-friendly, especially those with an adjustable gas block that allows you to control the amount of gas directed back through the gas tube.
6. Both piston-driven and gas impingement guns are very reliable. If, by magic, you inserted me in the pages of Bryce Towsley’s book The 14th Reinstated and I had to live through a social and economic collapse where there were roving bands of marauders, and if you told me I had to pick between a gas impingement or a piston-driven AR, I really would not care which one I ended up with. Except for three things: parts for gas impingement ARs are easier to find, much more plentiful, and less expensive.
Now, here’s the good news. The unmatched modularity of the AR allows you to, in a way, have your cake and eat it, too. If you own a gas impingement AR and want to try a piston-driven AR, just purchase a piston-driven upper receiver. Since the gas impingement and piston systems work independently of the lower receiver, you can alternate between both on the same lower receiver.