Have you "miked" the depth of the rim recess cut into the chamber and compared it with the rim thickness of the brass you are using?  That might give you a little insight into what is moving and what isn't moving.  I do know that on my Remington Creedmoor rolling block, and some of the .43 Spanish rolling blocks that Remington produced, the recess for the rim in the chamber was cut .090 deep.  I finally did get some 44-77 and .43 Spanish brass from Track that was made with the correct rim thickness. 

I would have to go back and reread my book on gunsmithing the single shot rifle, I believe De Haas said that anything over .002 is excess headspace.