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
Doesn't seem like an improvement over my WSM or RUM (basesd off of the 404 Jeffery) idea as once fired cases can be found by the bucket load at most rifle ranges. .500 or .512 can both be used although the .512 will have less case taper. I have even necked down shortened WSM and RUM cases down to .458. Bottom line 500 Jeffery cases are more expensive and are not as readily available with no improvement in strength. My 458GXT has case capacity between the 45-70 and 458 WIN MAG is a necked down RUM case with a OAL of 2.800 (308 Win length) which is on the same concept as the 500GXT. I think WSM/RUM style cases would be a better choice and worth investigating on your end.
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Haven't decided on a rim size.
500GXT, 458GXT, 45-70 500SM, 458SM
Ive been eye balling the WSM and Rum cases a lot as well. there are a ton of posibilitys based on all these some what new short mag cases. or just use a 308 length gase to begine with that already exists. LOL