I bought this for a JG BAR-10, and it's been great so far.
I had a bad run-in with a 190 spring, so currently this piece is being shot by the stock JG spring, at about 420 fps.
It stuck with me through the hard times as I battled with an overpowered spring and was always cooperative. Now it shoots just fine with the stock spring, though it doesn't seem to agree much with the stock sear set (rough catch).
However, contrary to what I was seeing in the older reviews, this piston did not come with a spare -ring,yet don't view this as a pro or a con as it doesn't mention anything about that in the description.
All said, it's a great piston, at the best price around.
Slightly higher FPS
fits easy; o-ring doesn't fight it
Catches a bit rough on the stock sear set. I haven't upgraded my sears yet (due to my lost battle with the 190 spring), so perhaps this is remedied with that?