The question still comes up and the debate rages on- “Should I Get a Piston for my AR or Direct Impingement?” The answer is, the AR already has a piston and none use a direct impingement system.
First, let’s take a look at a conventional Direct Impingement system. The key feature is that gases act directly on carrier. The gas travels through a gas tube to act directly on the carrier. The following diagrams show the DI system of the Ljungman Ag m/42.
Eugene Stoner said in the original patent for his gas sytem “This invention is a true expanding gas system instead of the conventional impinging gas system.” The gases enter the expansion chamber inside the bolt carrier and drives the carrier rearward. The piston is actually the tail piece of the bolt. The rearward motion continues, unlocking the bolt and extracting the spent case.
The system is designed to be self regulating. When enough pressure enters to operate the action and the carrier begins to move, the key separates from the tube and cuts off the gas flow to the carrier. There are also vents in the carrier to allow the gases to escape and the pressure to drop inside the cylinder when the piston seals move past them. After that, the action is powered by momentum and returned into position by the energy stored in the action spring.
In the AR system, Eugene Stoner eliminated the actuator rod of other piston gas systems and incorporates the piston with the bolt and the cylinder with the bolt carrier to “…provide smoother operation and longer life of the working parts…” The patent goes on to explain that since all the actuating force is inline with the bore and bolt to the shooter’s shoulder “all of the off center loads found in most other types of gas actuated weapons are eliminated” to cut down on “climb” during automatic fire.
The so-called “piston upper” (ie- Adams, HK416, LWRC etc.,) does not convert an AR from direct impingement system to a piston system. It converts the AR from an inline piston system to an offset piston system and adds an actuator rod.
Anyone tells you your AR needs a piston to run cleaner/cooler/tacticooler, tell them
“No thank you. My AR already has a piston”
It’s always nice to pick up more info on the firearm we love so much, great write-up. Strap up for all the naysayers’ comments…they’re sure to come. :rolleyes:
I’ve always thought the name “Direct Impingement” was unfortunate. A more descriptive name would contain the word “coaxial” as this is the feature that truly distinguishes this system from all the others. Perhaps “coaxial piston” or “coaxial system” or some such. It is indeed true that all ARs, indeed, all centerfire auto rifles except the handful of recoil operated systems, are piston systems.
There was a post on TOS about this also, I believe by Steve from adco. (there are 2 "bigbore"s over there so im not sure which one wrote this). It has cut-section pictures to help people see what actually is going on.
Well, the term “coaxial” implies that the axes of two components (in this case the barrel and the piston) are parallel, but not necessarily that the axes are the same. I think a better description of the Stoner system is just “axial” or, even better, “in-line.”
The actual patent is a great read. In the description, Mr. Stoner states: “This invention is a true expanding gas system instead of the conventional impinging gas system.”
In light of this, I will never again refer to my 6920 as a direct-impingement weapon.
More precisely, it says “This invention is a true expanding gas system instead of the conventional impinging gas system.”
It doesn’t matter if the AR is a direct impingement system or not. The point being is it does have a piston (and I suppose push come to shove, so does the conventional DI system) and that the offset piston upper adds an actuator rod and introduces off center recoil.
Critics of the AR inline system talk about tight tolerances. Yet the carrier has little contact with the upper receiver. The tight tolerances are between the bolt and the carrier.
Perhaps someone with more experience with the AR can tell us where most of the crud and debris collects in the AR and what the direct effects are. I read statements that carbon build up on the tail cone of the piston doesn’t matter. I can see where debris in the barrel extension would prevent the bolt from locking up properly. I also know that keeping the action lubed will keep it running.
The offset piston system can produce tough and reliable rifles. After the AR, Eugene Stoner went on to engineer other weapons using offset pistons. But his design that went on to be successful is the inline piston of the AR
Also carbon buildup doesnt make a diff unless its in excess, like 5000 rounds. In the military you wont even come close to that before cleaning. A combat load is generally 7 magazines.
Wipin down the ar-15 once a day and lubin will keep it running
I didn’t mean to imply the AR isn’t. I read reports from the first Gulf War that the AR proved to be the most reliable of the 5.56 NATO rifles that saw action. One advantage the AR has is that it’s spent more time in the crucible of combat in the harshest of conditions than almost any other rifle
Also carbon buildup doesnt make a diff unless its in excess, like 5000 rounds. In the military you wont even come close to that before cleaning. A combat load is generally 7 magazines.
Carbon buildup is not the only issue for mil guys.
Sand from weapons being exposed to the elements, dirt and vegetation getting into the action or getting on the mags in the pouches. Snow and ice getting into the action.
I had a guy whose weapon malfunctioned during a live fire ex, squad hasty attack. Bolt wouldn’t cycle, and we couldn’t lock the bolt to the rear. It was stuck, barely out of battery. With a leatherman and some brute force, we finally managed to get the bolt free.
Upon inspection of the barrel extension and inside the reciver, we saw small pebbles, the size of coarsly grated pepper. They locked the action up.
During one operation a team had several guns fail to function after a while, during contact; HK416’s, MG-3’s and M2 .50 cals due to mud and dirt getting EVERYWHERE.
