When researching or purchasing an AR-15, you may come across the terms Direct Impingement or Piston Driven. This relates to the type of gas system an AR-15 (or AK-47) rifle or AR pistol uses.
Direct Impingement (DI) Gas System
Most AR-15 rifles and pistols on the market today are DI. That means whenever the trigger is pulled, there is a small explosion that takes place when the firing pin strikes the primer of the cartridge, and the gases send the round through the barrel to its target.
The gases go down the barrel until they get to the gas block and gas port which redirects the gases (like a u-turn) to the gas tube back down to the gas key and into the bolt carrier group (BCG). From there, the gases propel the BCG back into the buttstock where the buffer and buffer spring push the BCG back into place picking up another round in the chamber. That’s how DI gas systems work.
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There are two different types of piston-driven gas systems: Long Stroke (like an AK-47) or Short Stroke (like many AR-15’s).
Long-Stroke Gas System
Again, when the trigger is pulled, there is a small explosion that takes place when the firing pin strikes the primer of the cartridge and the gases send the round through the barrel. However, in a long-stroke piston-driven system, the gases go down the barrel and the gas block catches the gases but it doesn’t redirect the gases down the gas tube. Instead, the gas pushes a cup connected to the piston rod, which then reciprocates the BCG with just enough gas. The rest of the gas bleeds out through little ports on the gas key. A benefit of a long-stroke piston system is less felt recoil due to gas in the barrel being allowed to dwell longer which lowers the amount of gas going through the gas operating system.
Short-Stroke Gas System
Short-stroke systems work similarly to the piston operating system in long-stroke guns. One of the major differences is short-stroke systems are primarily used in carbines that don’t require a traditional BCG and Buffer system. The recoil system is usually housed completely within the upper receiver.
DI versus Piston Driven AR-15: Pros and Cons
In most cases, DI gas systems are less expensive than piston-driven systems. DI guns also have fewer moving parts which result in less recoil and a lighter gun. Piston-driven gas systems require more parts which typically increases the cost, weight, and recoil over DI guns.
One of the benefits of a piston-driven gas system over a DI system is that it shoots cleaner. There is less gas and carbon build-up throughout the barrel, bolt carrier group (BCG), and trigger because the gases escape through the front of the gun, away from the shooter, and outside the gun. There are no ports on the BCG because there are no gases coming through the BCG from the gas key. With a DI gas system, the ports in the BCG are necessary to release the gases from the gas key.
If you like to shoot suppressed, a DI system slows the gases down in the barrel and suppressor causing them to blow back toward your face (be sure to wear glasses). However, you can purchase an adjustable gas block for your DI system to adjust the gases.
With a piston-driven system, the gases are already going forward through the barrel and suppressor so they don’t blow back into your face. And piston-driven systems sometimes have an adjustable gas block to tune the gases and felt recoil.
Piston-driven gas systems run cleaner and may have less felt recoil. They tend to be more expensive and heavier (some newer piston-driven systems are a lot lighter nowadays). DI guns are lighter and more affordable, but they are dirtier (clean them frequently).
So What’s the Best Gas System for You?
It depends on how much you can afford, how often you plan on cleaning it, and what you plan on using it for.
If cost isn’t a factor, then a piston-driven system may be a good choice if you don’t like to clean your gun often or if you want to shoot it suppressed.
If you clean your AR-15 often, want to save money, and want something lighter, then a DI system may be best. And because you can purchase an adjustable gas block for a DI system, you can run it suppressed.
If you’re looking for a DI or piston-driven AR-15 or AK-47 rifle or pistol, join Big Daddy Unlimited today!
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