I've got an Adams Arms piston upper, mounted on a DPMS lower that I use for work. It had a bad barrel when I got it, but don't know if it was built that way or the dealer did a switch. The bore looked like someone shot 100,000 rounds full auto. Anyways, Adams' customer service replaced the barrel for free and now it's 100%. Last year I took it to a tac rifle class, where we shot probably 500-600 rounds in a day. It was very accurate and worked/cycled perfectly throughout the class. It was nice to not have to clean a bunch of carbon off the bolt areas because of the piston system. I'd buy another one.