Sad to say I still haven't cracked this. I first tried re-sizing the Walther seal, and managed to get the lip diameter from 25.29 mm down to 25.05 mm (the Walther seal seems very hard, and I had to attack it quite savagely to remove enough material). This gave me a "steady" push fit, much easier than before, but still fairly firm, and the energy only went up to 8.2 ft/lbs, so I fitted a 25mm Weihrauch seal, which gave me an easy push fit as supplied, but with 38mm preload the energy still only went up to 8.9ft/lbs. It does shoot quite nicely at this, but I still don't understand why I am getting so much less energy than Mr Tyler, using, I think, identical parts.