Gas. It can cause blinding flashes, especially when firing your AR-15 with a muzzle brake. It is also a crucial element to how your AR-15 rifle functions. To avoid any critical mistakes with your 80% build, read on below!

Before we dive in any further, here’s what you’ll learn here today:

  • The most common types of AR-15 gas systems.
  • How an AR-15 gas system works.
  • Key components within the AR-15 gas system.
  • Why AR-15 gas system lengths matter.
  • The best adjustable gas blocks on the market today.

Two Types of AR-15 Gas Systems

Gas-recoil systems were designed to replace rifles’ need for a manual bolt – allowing for a much higher firing rate. For the AR-15, there are two types of gas systems to know: direct impingement and piston systems.

AR-15 gas systems

Direct Impingement (DI)

Whether you’re building your own 80% AR-15 or buying a completed AR-15 from a store, the chances of winding up with a direct impingement gas system is pretty high because it’s the most common. Originally designed by Eugene Stoner himself, this system redirects some of the gas used to propel bullet out of your rifle’s barrel and back towards the bolt carrier group to re-cock the hammer, eject the spent casing, and load the next round from the magazine all in one fell motion from just gas. 

By enabling the BCG to move backwards and forwards in this fashion it gives semi-automatic fire capabilities. 

Piston System

The piston system is an alternative gas system for the AR-15. Originally designed by Mikhail Kalashnikov for the AK-47, there are a lot of similarities between the direct impingement and piston system in that they both use redirected gas to push the bolt carrier back for case extraction and semi-automatic fire functionality. The key difference is that for the piston system, redirected gas is contained in a separate cylinder which actuates a piston that pushes a one-piece bolt and rod backwards which is connected to the BCG. 

Still not clear? Check out these great visual aids to see how the two gas systems work:

Pros and Cons?

The DI system has become what we in America are familiar with as “mil-spec” and for good reason. It’s reliable, battle-proven, and the amount of gas funnelled through to be redirected towards the BCG can be fine-tuned through an adjustable gas block. This allows for shooters to tune their AR-15 rifles to be optimized for a wide variety of ammunition calibers or even suppressor use.  

Piston systems do hold multiple advantages over the DI system. The piston system allows your BCG to run cooler and cleaner compared to BCG’s run in DI systems. It is so efficient that you can actually take out the BCG from a piston system and hold it in your hand after it was being used to fire several rounds and not get burned by it. This is possible because gas is directly pushed back into the upper receiver in DI systems but for the piston system any gas not used in the redirected effort to actuate the piston is released at the front of the barrel.

But, if parts break piston systems can be more inconvenient for gun owners because piston parts are not interchangeable between different manufacturers. Typically, a piston system and its parts will be proprietary to its original manufacturer.

Why AR-15 Gas System Length Matters

When picking out a gas system, you’ll note an abundance of choice. In this case, a variety of lengths. The length of the gas system you choose will directly affect how much recoil energy can be used to operate the bolt carrier group effectively. With too much gas in the line, your AR-15 will become over-pressurized which will increase unwanted recoil and wear down critical parts of your beloved AR-15.

Alternatively, too little gas in the system can cause “short-stroking” which leads to multiple catastrophic failures, including failure to eject, feed rounds or regular jamming — nothing we want to deal with at the range or in the field.

Generally speaking, here are some numbers to abide by when choosing a gas length for a DI system:

Gas System PistolCarbineMid-LengthRifle
Barrel Length<10”10” – 18”14” – 20”20”+
Gas Block Distance4”7”9”12”

Essentially, the longer your rifle barrel, the longer the gas system.

Top-Rated Adjustable Gas Blocks of Today

By adjusting your AR-15 gas block, you could reduce recoil, wear, and further customize your 80% rifle. Here are a few making waves on the market today:

  • JP Enterprises Adjustable Gas Block w/A2 Front Sight
  • Superlative Arms Adjustable Gas Block
  • Double Star Pic Rail Adjustable Gas Block

Do you feel like you have a better understanding of AR-15 gas systems? Are you ready to build an 80% rifle of your very own? Now is your chance to completely build out an 80% AR-15 before impending anti-2A legislation from the Biden administration comes to take that right away!