hi iv buttoned my hw99s on the front and rear [you don't have to do front but did mine while it was down] I hade to take a bit of metal of all my HW pistons iv buttond at the rear if you don't the buttons are very thin proberly about 6 thou thick, that's proberly why tunners are fitting bands in now . my HW99S was dead rough as std when cocking now its smooth and shoots ok but still not accurate [ bit of work to do on the barrel I think ] imo the 99s is not a good engineered rifle like the hw95 it has some issues .but everything can be sorted out with a bit of time
atb john