The secret behind the first full power Webley lay in the patented piston head used in the Vulcan. This was the work of the then technical director Harold Resuggan who later became MD. This was a composite head using PTFE with a recess and a soft buffer between it and the piston. The idea was that pressure would push the PTFE head towards the piston reducing the volume of the recess. A double whammy. To get the most from anew Vulcan it was best to dry fire the rifle a couple of times to size the PTFE head to the chamber.
Whether it workedor not is open to debate but certainly the Vulcan I bought was way more powerful that my MK3 and the pathetic output of the Hawk which used a composite piston ring.