Call Shawn Carlock at Defensive Edge. I've been going through the same thing and he'll help you out. He sells the dies and you can get high-quality 300 RUM Brass to Neck up to make it.
There are a lot of 300 RUM fans in this game but it seems like unless they are die-hards, most will sell a 300 RUM and move up in Caliber. the 338 EDGE is a whole differenct animal compared to the 300 RUM and the 338 EDGE fights wind and is just "THAT" much more consistent at longer ranges. You're burning powder out the identical case, I would heave the bigger projectile hands down.
..:efensive Edge - Custom Gunsmithing::..