The optimum for accuracy might be a system that would enable direct loading of pellets into the barrel, like some FT rifles. That is to say avoid the bolt action with transfer port and have a direct feed from the valve to some other  kind of loading mechanism behind the barrel like, for example the Gamo CFX.  I have been thinking of something like a falling block that would enable pellet loading rather like the old Martini action but have yet to come up with a suitable metal to metal seal.

Ok, I've re-drawn the lot in AutoCAD now and have a 12 mm hole for a valve module, still on the big side. What thickness of the valve walls would be adequate in a brass valve of 6 mm? ldurham, would you like to "own" the valve module for now?

 /Mike S