Try the NEI handtools sight. They list several up to 645 gr. They also show a design specifically for the Winchester Magnum chamber. I use an RCBS paper patch 550 gr in a 458 WM. I also have a Saeco 540 gr mould that works well. Both feed from the magazine and the paper patch is surprisingly accurate. I think a bullet that filled the throat would have to be loaded single shot and would not fit in the magazine.