Excellent review TMS951. I have only shot a couple magazines of SP ammo through mine and found the same issue, but only with the last two or three rounds from each magazine.
I also noticed the same copper fouling and stiffness with the piston sleeve, although mine has never seized, even after several hundred rounds shot in a single session. After I initially noticed the fouling, I just remove the powder and copper build-up and add a light coat of oil to the sleeve while I am cleaning the rifle. This afternoon I ran drills for about 420 rounds and took a picture of my sleeve showing what I see. 360 rounds suppressed and the remaining unsuppressed.
I have also noticed a small amount of erosion on the gas port on the "Normal" side of the sleeve.
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