I necked down the 500 Jeffery case to get an idea on what it would look like, the neck will be longer of course, I left it short so I can measure the chamber first to make an adjustment to fireform with proper headspace..... the shoulder will also be fire formed up to 35 degrees...
necking down wasn't as difficult as it would appear to be, I had most of the necessary dies to gradually reduce the diameter of the neck, after determining the proper diameter I bought a couple more die sets to get the proper squeeze....
I just used the bottom portion of standard caliber dies with the appropriate diameters to reduce the neck with zero case losses.... (lotsa Imperial wax)
500 Jeffery .532" neck OD
500 S&W .530"
300 Win Mag .513"
6.5x55 Swede .480"
454 Casull .477"
30-06 Springfield .470"
44 Magnum .457"
220 Swift .445"
6.5 Grendel .437"
40 S&W .424"
30-30 Win .419"
25-35 Win .416"
5.45x39 .394" final die used and 330 gr. Cutting Edge bullet seated with good tension
I realize that is a lot of steps and a lot of dies.. but it worked and I gradually reduced the huge neck opening down to accept a .375" bullet with no problems, I have a batch of brass ready to fire form and did not need to purchase expensive case forming dies ( $500 and 6 month wait was the quote )
Now I have $500 saved to build a real set of dies from my fired cases !!!
Next I will work on continuing to reduce the neck to accept a .338 cal bullet...
additional dies required
9mm Luger .391"
9x18 Makarov .384"
357 Magnum .379"
30 Carbine .354" ready to seat 338" bullet
gotta get me the 30 Carbine and 9x18 Mak dies and ready to go !
I am always amazed when guys are excited to shoot cartridges of this magnitude. The muzzle breaks used must be quite effective??? I own break less 8MM Mag and 340 WBY and consider those quite substantial as far as recoil is concerned. Holy crow! Good luck with this. I was hoping to see more happen with the 6.5 Prometheus as well.
The 6.5 Prometheus project is alive and well, have the rifle completed, waiting for Neil Jones to finish up my dies and will start load development when dies arrive, have a fluted 8.5 twist BRUX barrel to burn up with load developement and fireforming brass and to see how long the tube will last, then will throw an 8 twist Bartlein bbl on. I'm hoping throat life will be a bit better with the long neck and 35 degree shoulder, we shall see.....