BSA Standard Pattern struggling to make good power

[jirushi]

Traditional British craftsmanship; I must practice!
However, a lot can be done with some diamond files, a small hammer, and a sixpack of Leffe!

Some news from Wouter:
after reducing the size of the leather piston seal, as kindly recommended above, velocity went up from 440 to 490 ft/s.
Not bad. Not good enough though, imo. Also, Wouter now describes the gun as twangy.
I recommended to drink a 500 ml Bavaria beer (absolute cowpiss, but it comes in a steel can) and use that for a piston sleeve. Perhaps not the most durable solution, but it worked in my Meteor.

Could his spring be too long/strong? Is there something like an oversprung gun giving less power than when a shorter, correct spring is fitted?

Also, he hasn't used neatsfoot oil yet. Perhaps that will up power; yes, it must make the leather softer, and thereby increase its sealing capacity.
I think