I would try a .640 to .655 for Hevi 7s and for Hevi 6s or Win. Xtended Range try a .655 to .665 diameter
If trying the Federal with flight control shells in Heavyweight I would only try the #7s. Just no enough pellets in the 6's. Federal recommend a non ported tube, but some here have used ported and had good results too.
These were the findings in my SBE2. Hope my advice saves you some cash.