I had a CMMG piston upper over a Rock River lower. One day at the range the weapon stopped feeding rounds. Basically it turned into a single shot AR. When I got home and disassembled the upper I found that the piston rod return spring had jumped over it's retainer therefore failing to push the piston back into it's bore for the next shot. It was at that point that I decided piston ARs were not for me. Didn't think to take photos. Sorry bout that, they would have been interesting to look at now.
FFL Dealer
NRA Pistol and Rifle Instructor
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