Just a quick up date.
Have the gun torn down again.
You are probably right on the headspace issue.
I think I'm having a problem correctly measuring headspace.
I don't have the tools to accurately measure the cartridge rim, so I just used . 60 for the rim depth.
Then measuring with a feeler gauge, I get .012, which is how I arrived at the total headspace of .072.
The shims arrived yesterday.
Cylinder to barrel measures at .012
The yoke on the crane has very limited movement.
When I move the cylinder left to write there is a very noticeable gap between the crane/yoke and the frame in the closed cylinder position.