Well try a different shell holder with the full length die and see if that dosent help.
Sinclair sells a set of what they call competition shell holders for the purpose of bumping the shoulder back more.
Just follow the directions until you get to the one that allows the bolt to close.
But it could well be that brass is now junk.
If a guns gonna shoot well, its gonna also shoot with cases that are full legnth resized, and if it don't, how you size wont matter much.
If you want tiny groups you wont be shooting a magnum anyway.
Magnums have but one purpose, better performance for hunting at longer distances.