I recently shot all my guns to group them at 60 yards. Granted, I ain't the greatest shooter - but in my hands my Powerline beat everything else I had - whether .22 or .177. This includes an RWS Panther, a Shadow, a CFX, and an RWS48. Granted, the ones that grouped the best were all pretty close - well within my margin of error. But it shows that the gun can do far better than its price would indicate.
I substituted an O-ring for the breech seal, I believe it was a #108 or #109. I don't remember if I had to shim it. Never had to look for a piston seal.
As for the rest, I tarred the spring on this one - which made a BIG improvement in firing behavior. I also ever-so-lightly smoothed over the interface surfaces between the sear and the intermediate lever and applied a little moly paste - and the trigger now breaks at less than 2 pounds (which helps with the accuracy, I'm sure).
