From:
https://www.americanrifleman.org/articles/2010/4/13/ar-operating-systems-gas-impingement-vs-piston/Gun owners have strong opinions when it comes to Eugene Stoner’s AR design. There are those who decry the path of the “new and improved” gas piston guns as mechanically unsound and inherently flawed. On the opposite side are those who deride the traditional AR operating system as a poor design, self-fouling and jam-prone. Standing in the middle of the fork is everyone else, the undecided, trying to determine which way to turn.
The first and most important reference mark is the fact that the Stoner-designed AR with a direct gas impingement system is by far the longest serving military service rifle in U.S. history, . . .
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An Answer In Search Of A QuestionThe sole claim to fame of a piston system is that it’s more reliable than an impingement system. The reason given is that hot, dirty gases are not spewed into the action of the rifle like the direct impingement system, fouling itself with heat and carbon, depositing black crud all over the bolt carrier. Instead, the gases are contained in a gas cylinder which is self-cleaning.
No one asserts that piston systems are more accurate or more durable, just that they’re more reliable because the bolt carrier is not caked with fouling and subjected to scalding heat.
Advocates of the piston system are quick to ask, “Why would you dump hot, dirty gases where your rifle feeds?”
There’s no question that heat and fouling are highly detrimental to moving parts in a firearm. The solution, however, does not necessarily require redesigning the whole gun. Fouling problems can be avoided quite easily with a marvelous little thing called lubrication.
“Keep her wet. That’s how you run an AR,” a hard-bitten range master once told me. “I don’t care what you squirt in there—BreakFree, WD-40, lime juice. It doesn’t matter. Soak her good.”
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More importantly, however, a piston system alters the mechanics and timing of an AR in a manner that a growing number of shooters are claiming is harmful to the gun.
There are new systems being developed, tested and marketed now, but generally the problem is that a piston system is attempting to retroactively adapt a bolt carrier that was designed to function with direct impingement.
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This protrusion is attached to the bolt carrier well ahead (toward the muzzle) of its center of gravity. Going back to see-saw 101, we realize that if a force is applied well in front of a pivot, what happens? The rear tilts.
This see-saw effect is causing bolt carriers to tilt within the receiver, retarding their movement and imparting a non-linear force to the assembly. Stoner did not design the bolt carrier group to be hammered.
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Direct impingement
http://truthaboutguns-zippykid.netdna-ssl.com/wp-content/uploads/2012/06/DIRECTM16.gif
Gas piston
http://www.hk94.com/hk/uploads/monthly_05_2009/post-1-1242084526.gif
