Gunk in my upper w/ AA Piston. ???

[No Bananas]

I have about 1700 rounds through my 10.5" LMT upper w/ AA kit installed. It runs like a top, and I've never had a jam. I've only cleaned it a couple times, because the bolt and carrier stay so clean. However, when I recently did a thorough cleaning, I noticed that the inside of the upper had this gray colored film/gunk on it (no, it is not the dry lube) that was incredibly hard to get off. I really had to scrub with a copper brush. When I take it off on a rag or shirt it is dirty midnight black. I use slip 2000 EWL on all my guns, I don't get this build up on my other DI ARs. Anybody else have this experience? The bolt carrier is melonited, could that have something to do with it?

[Heavyweight]

It's still carbon....you're getting blowback through the chamber. Piston ARs don't do anything for gas coming through the chamber. Are you running a suppressor? If so this is normal. If not it could be that your bolt is releasing from the chamber too soon allowing the gas to blow back. The AA bolt is spring loaded in the carrier to assure lockup as long as possible once you fire. I would disassemble your carrier and bolt and clean them thoroughly....when you reassemble your BCG and press down on the bolt it should spring right back out of the carrier nice and smooth. If your bolt is still not springing back well you could need another bolt spring from AA.

As you already know it doesn't affect function....just a little longer cleaning time!

Good luck!

Heavyweight

[ssracer]

What type of ammo are you using? If its steel cased, that could be it. As I understand it, since the steel doesn't expand the same as brass it will allow more residue to blow back past the case.

[No Bananas]

The springs works as it should. Bolt bounces right back with strong force. I don't shoot any steel cased ammo. I shoot Q3131, Q3131A1, PPU M193, some Fed Gold Metal:D, only a few boxes of Am. Ea., and mostly PMC Bronze 55 gr.

[Heavyweight]

Most pistons are over gassed...try turning your gas selector one notch counterclockwise from twelve o'clock. That's where you would normally run with a suppressor...but mine runs ok on that notch without one. You're effectively slimming down your gas port and this will keep less gas out of the chamber. Downside is your rifle might short stroke but I'd give it a try.