* First, let's agree that you won't bust on me for using the "DI" and "piston" terminology, and I won't bust on you for using the term "AR" when you are not talking about Armalite rifles . . .
If Armalite holds the copyright on "AR" (just the letters "AR", not "AR10" like they do with their 308) , I wish there was a better way to refer to this family of weapons. But I don't have one.
Calling the AR system a "direct impingement" is like calling a turbine engine a rocket engine. It's not.
I know that the gas tube dumps gas into the action. That's why I pointed out the system vents also vents gas when the gas key separates form the gas tube.
I run piston in gas block rifles, a variety of them. I know where they dump fouling. I've gotten carbon fouling all over my support hand shooting them. The point about fouling is, I don't find one to have less fouling than the other. They just dump it in a different location.
A piston in carrier AR needs it's lubrication inside the carrier where the piston/bolt moves back & forth. Lube outside the carrier isn't nearly as important because the
clearances are actually rather generous.
The point I was making is, while the piston/oprod system has certain real or imagined advantages, the same is true for the piston in carrier system. Both systems work reliably. But the piston in carrier system isn't the boogeyman some make it out to be. (Of course the same is true of the piston/oprod system.)
Oil not only lubricates a machine, it also carries away dirt & fouling. It keeps dirt & fouling from gumming up the works. One problem with putting the piston in the gas block is that it's difficult to lube and if it were lubed, it'd get blown away by the gases very quickly. Now add to the fact that the piston in the gas block has to run dry, it's also subjected to hotter gases and has a smaller diameter to deal with it. One of the criticisms of having the piston inside the carrier is that it's subjected to "hot gases". Yet the gases are barely enough to warm the carrier or bolt! I shot my rifle until the gas block was hot enough to flash water to steam, yet their wasn't enough heat in the BCG to warm my hands with.
If a "hot" piston is bad, how is a hotter piston better? At least the piston in the carrier can be lubed with oil to keep the carbon soft.
It's time we look behind the curtain for the facts rather than parroting what we read on the internet or hear from gunshop commandos. Heck, when I first started learning about ARs, I was told it was pistonless direct impingement system that worked the same way as the Lljugman, that the pressure in the expansion chamber pushed the bolt forward to assist in unlocking and extraction and that the length of the barrel from gas port affected BCG speed because the bullet was in the barrel a half micro second longer. With a little research I discovered they were all fallacies, some even to the point of violating the laws of physics.
A piston in gas block system is a good one. It's proven reliable in several battle hardened rifles. But it's not the "savior" of the AR. It's no better or worse than having the piston in the carrier