If your talking the 338x74 Keith, it is made from the german 9.3x74R cartridge. I believe Ruger chambered the no.1 rifle for it at one time. If you have a copy of Gun Notes Vol 2 it is on page 11. Clymer made the reamer and Huntington(RCBS) made the dies.
Data:
Nosler 250 gr, 76 gr 4831, CCI 250 mag primer- 2711 fps
Speer 275 gr, 74 gr 4831, CCI 250 mag primer - 2580 fps
Win 300 gr , 72 gr 4831, CCI 250 - 2552 fps
Win 300 gr, 73 gr Norma 205, CCI 250 -2579 fps
This is Keiths original data published in 1971, use at your discretion. I believe the 9.3x74R is a currently produced cartridge, you should be able to find brass for it.