In this article, Robert A. Sadowski answers the question of What is direct impingement?
It happens in milliseconds.
Faster than you’re free to blink.
This illustration shows the direct impingement system including how it pushes the bolt carrier group rearward. Image: U.S. Army
This is the basic operation of the DGIS on standard AR platforms.
Egypt licensed the design and called it the Hakim rifle.
Not Really a DGIS?
Expanding gas flows through the key and into a chamber in the BCG. This movement allows the bolt assembly and bolt cam pin to unlock the bolt from the barrel extension. Image: U.S. Army
Here is where we start to split hairs.
Remember how the expanding gas is funneled into a chamber in the BCG?
Sorry to burst your bubble.
The French MAS-49 (MAS-49/56 variant shown) is an example of a of rifle using a true direct gas impingement system. Image: Rock Island Auctions
Direct Impingement vs. Gas Piston
So, what exactly is a gas piston system?
A common gas piston-operated rifle is the AK-47.
On an AK, it is a long stroke piston that is attached directly to the bolt carrier.
The bolt carrier key (shown affixed to the top of the BCG in this image) funnels expanding gas from the gas tube down into a chamber in the BCG.
There are pros and cons to both the DGIS and piston systems.
A DGIS AR-15 is a very reliable rifle, plus it is lightweight, accurate and produces less recoil.
you oughta clean the carbon from the BCG, but thats just part of maintaining an AR-15.
When a shot is fired, a port or small hole in the barrel vents hot, expanding gas upward through the gas block and into a gas tube along the top of the barrel.
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