This is an interesting cartridge. The Phantom is based on a cut-down .500 Jeffrey case. The Jeffrey's other name was 12.7x70 Schuler.
The main difference between the Phantom and the Beowulf is that the Phantom takes .510-.512 bullets instead of .500-.501 bullets. This means bullet selection is muc h broader than for the smaller Beowulf bullet.
The Phantom is also longer than the Beowulf, and is designed to feed out of a .308 magazine. It will feed (single stack) from an AR-10 or M-14 mag.
Recommended pressure is 50,000 PSI.
Brass can be cut down from Jeffrey or Schuler cases, or headstamped Phantom brass is available. CH4D sells sizing dies. Manson has a reamer print.
Feisty Rooster's 500 PHANTOM Page