Thus far in this series, we’ve covered the heart and soul of the AR-15 — that is the Bolt Carrier Group and the Barrel — and what to look for to make sure you’re getting quality parts.
But there’s one more part of the AR-15 that is important enough to devote an entire article to …
That’s the Gas System.
Why is the AR-15 Gas System important?
The gas system in an AR-15 is what runs the gun. Without the gas system, you’d have essentially a bolt-action rifle in your hands.
The original design of the AR-15 platform was for a direct impingement (DI) gas system, which we will focus on here.
Why not a piston gas system? The list of reasons is beyond the scope of this article but for now I’ll just say the DI system is just as reliable, less money, lighter weight, lower recoil, and has greater part interchangeabiliy and it’s easier to find replacement parts because there’s actually a standard.
With that said, here’s how the DI system works (in a nutshell) …
When you fire a shot from your AR-15, gas pressure builds up inside the cartridge case expanding the case and holding it in the chamber while the internal pressure from the cartridge launches the bullet down the barrel.
As the bullet went out the barrel, the gas that propelled it out escapes through the hole in the top of your barrel called the gas port. That gas continued through the gas port, the gas block, down the gas tube and into the gas key on the top of your Bolt Carrier Group (BCG).
This causes the BCG to start unlocking from the chamber and start moving backwards, pulling the now empty cartridge case from the chamber and ejecting it as it moves back past the ejection port. After the BCG slows and cycles forward, it strips the next round off the top of the magazine and chambers it, readying the cartridge for your next shot.
As you can see there are a lot of parts working together. The full-auto BCG we’ve chosen allows the BCG to do its job, so let’s talk about the gas system that powers this thing …
The 3 most common gas system lengths and which is the best …
There are essentially three gas systems to choose from with the modern AR-15. The rifle length, mid-length, and carbine length.
Here’s the key difference between these systems: the shorter the gas system the more system pressure is used to cycle the rifle. And the shorter the “cycle time”.
Theoretically, the longer the gas system the more reliable the rifle because of the A.) longer cycle time giving the BCG more time to do its thing (even if dirty) and B.) less wear & tear on the parts …
1.) Rifle Length. This is normally found on the 18-20″ barrelled rifles and is the original design by Eugene Stoner. This system has the best performance and highest reliability of all lengths, but it requires the longer 18-20″ barrel. Commonly used by the military with M16 rifles.
2.) Carbine Length. This is the modern design used on military M4 carbine rifles. It’s the shortest you can go and still be reliable because the M4 has a 14.5″ barrel. Of note, even the shorter 10.3″ Short Barrel Rifle (SBR) platform used by some special teams in the military use the carbine-length gas system.
3.) Mid-Length. This is the “in between” size of rifle and carbine. Because there are federal laws making anything shorter than a 16″ barrel a short barrel rifle (yes, even 14.5″ … stupid huh?) — the 16″ barrel is arguably the most popular civilian rifle for a carbine length choice.
In short, for a civilian legal “carbine” with a 16″ barrel, the mid-length gas system delivers the Jack-of-all-trades, best of both worlds, compromise.
It (should) have a higher reliability and lower parts wear than the carbine length system … and … much of the reliability and lower-recoil of the rifle-length system without the downsides of the longer barrel.
Why recoil reduction matters…
The fact is the 5.56/.223 is not a hard-kicking rifle, so you might not consider recoil reduction to be important. But these same parts all work together to reduce recoil and it doesn’t matter how “tough” you are, a low-recoil rifle is more desirable.
For one, it’s more accurate because it makes the gun a “flat shooter”. Plus, the less recoil, the faster you can engage multiple targets or get your sites back on the same target for the next, more accurate shot.
This is especially obvious when you are shooting using a magnified scope and each shot moves your crosshairs off target and you have to “find” your target again each time.
And let’s not forget that your children or maybe a female shooter might have to use your rifle to defend your home one day. You want a rifle that shoots flat, accurate, and with little-recoil so they can be as effective as possible with the weapon.
For all these reasons and more, the 16″ barrel with a mid-length gas system is quickly becoming the new “standard” for a quality AR-15 rifle.