The .500 Phantom is pretty much dead. Marty has DCed that round and there will be no more brass available. I am currently working with this round in the LR-308 platform and shooting the 649 gr. BMG bullet I am getting 925 to 960 fps. Very accurate too but not enough gas with the powder I am using to operate the action so it is back to the drawing board with another powder.
If you choose to use this round you will have to find someone to cut down .500 Jeffrey cases to spec and it is ashamed it is a dead round. It really packs a wallop and would work through a .308 length action, since that is what I am working with. The brass is the hard part and in a bolt gun it would be cake to work with, only thing is since the round is dead and no more brass is available where is one to get the brass, and I sure won't turn loose of any of mine.
A German company, Hornburer or something like that and imported through Garber is the one who made the last brass but it took a huge order and over a year to fill, which is why the round was allowed to die. The company was a real PITA to work with requiring all money up front and no guaranteed delivery date.