Cheers Mick-- give you a shout when I get sorted with a place.
Part of the thing I was on about was that the width and volume of the Webley breech seal should be taken into consideration in a calculation, but it looks like you have done that.
As a bit of an experiment Mick, could you try a Ptfe seal with a smaller hole in it and let me know the difference in fps or ftlbs, compared to a standard seal? I know you say that it wont last I would just be interested as to what it does, because I found that it moved POI on the 2 guns I tried it on and it SEEMED to be hitting the backstop harder (No chrono obviously)
I think the reason why Webley,s have the breech seal on the cylinder is something to do with on old airguns that had this set up, you could take out the breech seal and fit a firing pin to fire live rounds. Everybody went over to a seal on the barrel but Webley stuck with the seal on cylinder set up but choked the barrels to stop you doing this. pretty sure I read this in an article in AGW about 1980 It may have been something to do with an air shotgun?
Re the Diana 22/23 small Webley rifles, I think on the Diana 23 version I had, the breech seal was very thin (about 1/2 th thickness of the 22). I think I made it up out of the tongue of an old boot
Ps the 23 is the proper barrel version of the 22, like a Webley Ranger.