Although I admit I am not familar with the HW90 I owned a HW80 that started suffering from nearly the same problem, over a relatively short period shots seemed falling slightly below the point I was aiming at. I tried all sorts swapped the scopes over, changed the silencer and even the washers on the shuttle all to no avail. Eventually I gave up and took it to a gunsmith who later phoned me and told me the rifle was suffering from "barrel droop", basically where the barrel is machined into the breech block there is a port that takes your pellet around this port there was a fibre washer that ma