I've just bought a .22 (RB9608) non working from a dealer. Apart from the usual buffer washer mess the piston head had seized I got it out and turned it down with hand file and cordless drill until it all went back in and is now doing 5.5.
Mine shoots high so I'm going to look into your pointers on the sights ( either that or shoot it at 30 yards) It now lives with my bought new .177.
Good write up