Bit late with these, but thought I'd post some pictures of the hop mod I did to my Webley. Followed the instructions on how to do it off the excellent Nagant tutorial that's kicking about on the net. Had to fiddle about with the o-ring I used, as initially it was blocking the BB in the barrel! Got there in the end, though, and it has much better range now.
^ This was taken shortly after I started. Small round file with the barrel locked safely in a vice.
^ Taken after glueing and trimming the o-ring. I used very fine sandpaper to scrape off the excess and make it flush with the outside of the barrel.
^ The results! I neatly wrapped a few layers of tape around this end to keep the barrel from rotating during fitting and firing. Seems secure enough, but time will tell (there are more complicated, permanent solutions to stop this though).
If I can do it, anyone can.