This book has a chapter devoted to special issues with belted magnums.

http://www.amazon.com/Precision-Shooting-Reloading-Guide-Brennan/dp/1931220123

From my reading of it and your report, your brass will last much longer and probably perform better if you only neck resize, not full length resize(until the extraction becomes hard), after fire forming to develop your best load and OAL.

Last edited by Tom; 05/15/15.