You might get a few rounds of 9.3X62 Mauser brass in a European brand. That brass is larger at the head than 30-06 brass.
You can form it to 30-06 and it should stay on center when it expands since it will expand much less.
You can also try wrapping new unfired case heads behind the expanded area (wrap on the solid head) with a full turn of Scotch Tape. That is enough tape to evenly enlarge the head of your cases by .006". Then fire it and you should have concentric expansion even if the head is smaller. Remove the tape. It is only needed for the first firing.
You can form it to 30-06 and it should stay on center when it expands since it will expand much less.
You can also try wrapping new unfired case heads behind the expanded area (wrap on the solid head) with a full turn of Scotch Tape. That is enough tape to evenly enlarge the head of your cases by .006". Then fire it and you should have concentric expansion even if the head is smaller. Remove the tape. It is only needed for the first firing.
