How did the flatness of the Federals compare to the other primers?  With equal loads mine were a bit flatter than CCIs.  In my case both were magnum primers.  My use of magnums has been pretty much limited to either small doses of RL19 or large doses of 50BMG powder.  In those cases, with cast, it had the opposite effect you're seeing and tightened my groups.  Maybe because the hotter primer got the powder burning in a more efficient manner.