Perhaps this will help? 7/64 is pretty small IMO. I would go with an 1/8" or a 5/32" flash channel.
http://gandersmuzzleloadingblog.blog...ent-shims.html
You want at least a .005" crush fit on the primer to be blow by free. Losing all those hot gasses is what makes BH209 extremely dependable and almost flawless.