i had a 12 ft/lb one and fitted a FAC v-mach kit in it,
did a few other things and the gun shoots very sweetly now at 22ft/lb.
the best ones to get would be an old gun thats been tunned by SFS ,venom or v-mach, but other than that it's very simple to do it yourself with any HW80 just by fitting a kit,wuld take about 45mins first time
this is a step by step of working on a HW80(called R1 stateside)
http://www.pyramydair.com/blog/2006/07/spr...powerplant.html
with the FAC kit your replacing the piston seal,spring,top hat and spring guide
Bryan