AR-15 Gas Systems

This guide goes over AR-15 Gas Systems. Read on to learn more!

We saw something recently on Instagram going over gas systems for the AR-15-style rifle. It briefly went over how adjusting the gas system (to introduce more gas into the system) can lead to better performance of your rifle. 

While this adjustment to your gas system to increase your fire rate and improve your split times may sound good in theory, in reality, it isn’t. We don’t believe that creating an over-gassed system makes your rifle better overall.

What everyone should strive for when choosing or building an AR-15-style rifle is accuracy, reliability, and durability. These three trump everything else in our opinion. You want a weapon system that is going to hit what you’re aiming at, whether it be short, intermediate, or longer ranges. 

You want a rifle that is not going to cease up and fail you at the range, while training, or in the field. And you want a rifle that is going to last you as long as possible with little maintenance outside of routine cleaning.

While over-gassing your system can cause long-term problems, if not immediate. Undergassing it will lead to failures that are immediately identifiable and having a weapons system that can’t even cycle properly is obviously a no-go. 

Having the proper amount of dwell time to allow for the smooth operation and cycle of your weapon, while reducing recoil and wear on your internals in our opinion is the best use of a direct-impingement system.

In this guide we will try and break down as best we can the differences in system lengths and how they may affect the performance of your rifle, and you as a byproduct. Gas and the manipulation of this energy are what make the entire system work. 

So knowing it, its effects, and how you can manipulate it is obviously beneficial to everyone from the best gunsmiths out there to the person just getting into shooting for the first time.

Direct-Impingement System

Gas and the manipulation of these gases is what cycles and operates your AR-style rifles/systems.

Order of operations for what drives and cycles your AR:

  • Bullet is fired, and as the bullet travels through the barrel, gas/pressure created from the cartridge travels with the bullet through the barrel, exiting the barrel as the round does.
  • As the bullet passes your gas block, some of these gases enter the gas block through a small hole.
  • These gases travel through the gas block, into the gas tube, and through the gas tube back toward your chamber.
  • The gases then enter the gas key which is part of your BCG (bolt-carrier-group), which drives the BCG rearward.
  • This causes the bolt to rotate and unlock from the barrel/chamber, which then drives the action/cycle of your AR-15-style rifle to extract the spent casing, and chamber a new round to be fired.

This is the most common gas system you will find in AR-15-style rifles/platforms.

Cycle of Operations

AR15 Gas Systems

Gas Tube/System Lengths

  • This is where most of the debate occurs regarding this specific part of your AR-15-style rifles/builds.
    • Over-gassing a gun, under-gassing a gun, dwell time, acceptable recoil, etc.
    • Finding the gas system length that best optimizes your specific gun’s accuracy, reliability, and durability should always be the goal.
  • Gas System Lengths
    • Pistol Length – 4″ gas block to chamber distance
      • 10.3″ inch barrels and shorter
    • Carbine length – 7″ gas block to chamber distance
      • 11″-13.7″ barrels
    • Mid-length – 9″ gas block to chamber distance
      • 13.9″-16″ barrels
    • Rifle length – 12″ gas block to chamber distance
      • 18″+ barrels

**These are our recommended gas system to barrel lengths. We will go over this more in the next slides.

Determining Proper Length

​​When choosing a new AR-15-style rifle, or building your own determining the gas system is almost as important as determining the barrel size.

Like choosing the proper barrel length, choosing the right gas system will be influenced by the type of training/shooting and the role you want for it.

An inefficient gas system can lead to a ton of issues, malfunctions being the main one, but even after that, an inefficient gas system could be affecting the way you and your rifle perform.

Gas systems can affect everything from your gun’s cycle of operations, the amount of recoil your rifle puts out, and even the velocity and energy of your rounds.

The most important thing to know when choosing the gas length you want is determining the ideal dwell time

Dwell time is the amount of time a fired bullet spends in a barrel after it passes the gas port.

This specific moment in time is when pressure from expanding gas builds and some of the gas is redirected through the gas port into the gas tube.

If there’s too much dwell time (large distance between gas block/muzzle), gas will build up and over gas the system.

An over-gassed system will slam the bolt to the rear with much more force.

This will cause excessive recoil, can wear down parts quicker, and can lead to malfunctions.

Too little dwell time (short gas port to muzzle distance) leads to a gun being under-gassed.

Not enough force to send the bolt to the rear (known as short-stroking) and will lead to malfunctions.

Simply put, an over-gassed system will generate faster cycling, which is ideal for an increased rate of fire

But this will also lead to higher recoil, affecting the accuracy of follow-up shots, in addition to wearing down your parts at a much higher clip.

Under-gassed systems will lead to malfunctions due to not enough energy being present to maintain the cycle of operations needed.

This can also affect the velocity/energy of rounds.

Other Factors

Adding a suppressor to your rifle greatly affects how the gas system operates/functions.

Adjustable gas blocks- these allow the user to manually adjust the amount of gas that travels into the system.

Great for when taking on/off a suppressor.

Gas port diameters on your gas block can also help determine or fine-tune this.

Buffer systems- A heavier buffer/buffer spring can help reduce recoil for a heavier-gassed system.

Too heavy can lead to similar malfunctions to having an under-gassed system.

Ammo type- Heavier (higher grain) bullets will travel slower through a barrel and lead to longer dwell times.

All of this may seem like a lot but understanding your rifle and how different parts can affect its performance is vital when deciding which is best for you and your needs.

Can also help with determining inefficiencies/issues.

Buying from high-quality manufacturers who pay people to figure this stuff out will cut out most issues people may have with their rifles.

Piston Gas System

There is another gas system called the Piston System.

Gas is captured inside a separate piston rod, which then contacts the BCG and forces it rearwards to complete the cycle of operation, and extract/reload a new round.

The advantages of this are most of the gas/fouling is contained inside the piston system and does not get shot back into the chamber.

Easier to clean/maintain. Considered more reliable. Less wear on your internals.

Disadvantages of this type of gas system

They are more expensive, complex, and heavier.

Piston systems also are adjustable allowing the user to fine-tune the amount of gas entering the system

Great for running suppressed/unsuppressed

And for when switching between supersonic/subsonic rounds.

AR15 Gas Systems
AR15 Gas Systems
AR15 Gas Systems

Final Thoughts

Thanks for making it to the end of this guide. 

Be sure to read our other guides here!