Since Armalite introduced the first AR-15, there has been debate about the gas impingement drive system.
Colt began experimenting with piston-driven AR platform rifles shortly after they acquired the rights to the AR-15 rifle from Armalite. This is not a new debate, and it is one that will likely keep gun enthusiasts arguing for many years to come. Before joining the debate, it is important to understand the principles and features of gas piston vs. direct impingement systems.
The original design of the direct impingement system on AR-style rifles remains almost unchanged from the original design by Eugene Stoner. However, the first rifles fielded by the U.S. Army suffered serious reliability issues that were often attributed to the direct impingement system. Several alternatives were proposed, including changing the mechanics of the system.
After Colt acquired the rights to the AR design, engineers began experimenting with a gas piston system to replace the direct impingement gas system on what was then designated as the M-16 by the U.S. Army. Gas piston systems share some of the same design features as a direct impingement system but differ in how the energy is transferred to the bolt carrier group when the rifle is fired.
A solid understanding of the design and parts of each system is necessary before you can appreciate the advantages and disadvantages of gas piston systems and direct impingement systems used on the AR platform. Once you know these factors, you can make an informed decision about your personal preference as an AR owner.
Direct Impingement Systems
The original design of the AR platform uses a direct impingement system to drive the rifle’s operating system. This design is straightforward, which, for many shooters and operators, is a huge positive factor.
Simplicity on any equipment your life may depend on is often a good choice.
The Parts of a Direct Impingement System
All AR rifle designs, whether direct gas impingement systems or gas piston systems, share some common features. There are some subtle differences in these parts, depending on the gas system used to drive the bolt carrier group during firing, but they look and operate relatively the same.
The Upper Receiver
Every AR platform rifle has an upper receiver. The copper receiver houses the bolt carrier group and is where the rifle barrel attaches.
The gas tube is considered part of the upper receiver build on direct gas impingement systems.
Typically, the gas tube is hidden under the forward hand guard that surrounds the barrel and provides a secure handhold on the rifle during firing without the chance of contacting the hot barrel.
The Gas Tube
The gas tube is the heart of any direct impingement-driven AR platform. The barrel on a direct gas impingement system is drilled on top at a specified point to provide a gas port for the system.
A gas block fits around the barrel to harvest gas when the rifle is fired and directs it through the gas tube to the bolt carrier group in the upper receiver.
The gas tube fits between the gas block and the upper receiver. The gas tube mates with the bolt carrier key when the bolt carrier group is in the forward and locked position. The captured gas from the barrel exits the gas tube and drives the bolt carrier group backward to initiate the ejection and reloading cycle of the rifle.
Barrels, Barrel Length, and Gas Port Placement
If you are building a custom AR rifle, you will find that the selection of a barrel for your rifle is one of the biggest (and often most confusing) parts of the build process.
AR rifle barrels come in various lengths, rifling twists, diameters and profiles, and with different gas port placements. Choosing the right combination of features often depends on how you intend to use the rifle, which ammunition you intend to shoot most frequently, and your personal preference.
A standard 16.5-inch barrel with a 1:7 twist and a carbine-length gas tube design is the prevalent choice for most people. This barrel choice is arguably the best combination for semi-automatic rifles designed for common civilian uses such as target shooting and hunting small game or even deer.
The Bolt Carrier Group
At the heart of any AR are the bolt carrier and its associated parts. This collection of parts is collectively known as the bolt carrier group and contains key elements of any AR rifle operation. Most notably, the BCG has these parts:
- Bolt Carrier
- Gas Rings
- Firing Pin
- Cam Pin
- Firing Pin Retaining Ring
- Gas Key or Carrier Key
On a direct impingement system, gas pressure from the re-directed gas delivered by the gas tube to the gas key forces the bolt backward. This causes the spent cartridge to be ejected, the hammer re-cocked, and a fresh cartridge taken from the magazine and inserted into the chamber on the forward stroke.
Gas Piston Systems
Gas piston systems were designed to overcome some of the shortcomings of a direct impingement system. However, you don’t solve one set of problems without often introducing an entirely different set with which you must deal.
Gas piston systems are typically more complicated and involve more moving parts in the rifle, which can be a source of additional concern.
The Parts of a Gas Piston System
Visually, gas piston AR systems differ very little from direct impingement systems. It isn’t until you start disassembling a gas piston AR system that you begin to see the differences in how a gas piston gun differs from a direct impingement system.
Examining the parts gives us a much clearer understanding of the differences and how they affect the operation of an AR rifle.
The greatest difference between a gas piston upper and a direct impingement upper is how the gas’s energy is transferred to the bolt carrier to cycle the action. Instead of a gas tube, piston guns drive a rod that actuates the bolt movement. In older gas piston systems, the upper receiver required a larger hole to allow the piston rod to access the carrier key.
Newer gas piston systems now almost all use a piston rod the same diameter as a standard gas tube, making it possible to convert a direct impingement rifle to a gas piston system. You may use the same barrel, but typically you must change the handguard and often the sight post when converting to a piston system.
The Piston System
Instead of a gas tube, a gas piston gun utilizes a piston system that rides on top of the barrel and replaces the gas tube and gas block of a standard system. The piston system harvests the gas from the barrel.
Instead of directing the gas into the upper receiver to operate the BCG, the gas pushes a piston and rod. This allows the hot gases to remain outside of the upper receiver.
For this reason, gas piston guns typically run cooler and cleaner than direct impingement guns. The gas system is usually shorter and requires less gas from the barrel to drive the system, which can affect the rifle’s performance.
A gas piston system requires a slightly different design than an impingement system.
The carrier key on gas piston BCG designs has a flat face and a shallower pocket than most direct systems.
This provides a stronger surface against which the piston rod pushes to drive the BCG backward during the firing process.
In most cases, all the other parts of a BCG on a piston gun are the same and require the same maintenance and care that the bolt on any gun needs.
The major difference is that the gas and the products of burning the powder stay outside the upper receiver, significantly reducing fouling and temperatures in the upper receiver and on the bolt.
What Are the Advantages and Disadvantages of a Direct Impingement System?
After almost 75 years of production, testing, and field use, the direct impingement system is a proven and refined operating system for ARs. There were some teething problems with the early models, but engineers and designers quickly adapted the system to reduce those problems.
The Largest Advantage
In my opinion, the biggest advantage of a direct impingement operating system is its simplicity.
There are no additional moving parts to break or fail during operation. The system is easy to understand and easy to maintain under most circumstances. Gas tubes tend not to break or wear out, so there is rarely a need to replace or repair a direct impingement system.
The Big Disadvantage
On the other hand, a direct impingement system does have its drawbacks. Since the gas tube delivers gas from the barrel inside the receiver to operate the BCG, it also brings in unburned power, carbon, hot gases, and other contaminants that often remain on the bolt and inside the receiver.
This fouling can eventually cause the rifle to cease functioning if not cleaned and maintained regularly. Anyone who shoots a direct impingement AR with any regularity is well aware of the problems that can be associated with carbon fouling and residue build-up when shooting.
The Advantages and Disadvantages of a Gas Piston System
The fouling of the BCG and upper receiver is one of the reasons that gas piston systems are so popular among many AR shooters. Gas piston systems keep the hot gases, unburned powder and excess heat out of the upper receiver and away from the BCG. However, when you solve one problem, you often create others, which may be true with piston-driven AR platforms.
A Huge Advantage
Less fouling, less heat, and less maintenance are all related advantages to a gas piston system.
Keeping the hot gas and products of combustion out of the confined space in the upper receiver and away from the BCG and chamber is a huge advantage. Your gun should run cooler and cleaner.
This doesn’t mean you don’t need to clean and maintain your rifle. You should still field strip and clean the BCG, chamber, and upper receiver as usual. You will find, however, that your cleaning chores are much less than with a direct impingement system.
More Than One Disadvantage
Unfortunately, you pick up some disadvantages when opting for a gas-piston AR operating system. As I have said before, solving one problem almost always creates a different problem or, in this case, more than one problem.
More Moving Parts
Gas piston guns are more complicated and have more moving parts than direct impingement systems. More moving parts always mean more things to break, go wrong, or fail.
A piston gun may have more reliability and maintenance issues than a direct impingement system. This is one of the hot topics in the gas piston debate.
More parts equal more weight.
This often changes the balance of a gas piston gun and affects the barrel harmonics, which can change accuracy significantly.
This doesn’t mean gas piston guns are less accurate. It means they may shoot differently than a comparable direct impingement rifle. You may also be lugging around a few more pounds while in the field.
Last but certainly not least is the cost factor.
Gas piston rifles typically cost more than a comparable direct impingement gun. This can be especially true if you are planning a conversion to an existing gun. Conversion kits for gas piston rifles can add significantly to the overall cost of your purchase.
Two Operating Systems – Your Choice
For me, the choice boils down to personal preference as far as a gas piston or direct impingement systems go.
I prefer the standard direct impingement system, even though it requires more maintenance. For me, cleaning guns is almost therapeutic anyway, so I don’t really mind. In the end, you much pick your style of AR operating system depending on your needs and expectations.