I have been planning one for awhile, just haven't got around to it. I have a 375 Ruger so cases are not a problem.
4D rents a reamer
4-D Products Rentals
The first step I took was to see what was necessary to neck down a 375 Ruger case to a 30 caliber. After calling one of the die companies (either Redding or Hornady I'm not sure) and talking with them (one of their techs is shooting this round), was lead into using a 338RCM Full Lenght Die for the first step and then a 300RCM neck die. Was told and believe that going from 375 to 30 caliber was too much for one step. Necessary that it be a Full Length 338RCM since the forcing cone going up into the neck was more gentle than a 338RCM neck die.
Anyway, it does work. So the process for me is:
1. Size brass with a 375 Ruger Full Length die without the expander
2. Next size with the 338RCM Full Length die set to size the neck only
3. Finally size the neck down to 30 caliber with the 300RCM Neck Die
30/375 Ruger on left, 375 Ruger on right
300 win mag on left, 30/375 Ruger in middle, 375 Ruger on right
Now I am going to ream the chamber on an existing 300 win mag. It is a Beretta Mato so the mag is plenty long and it has a Lothar Walther barrel on it already.
My purpose (and it may different than yours) is to have an unbelted 30 caliber that will push a 200 gr Accubond at an honest 3000 fps without a muzzle brake. I do not want to shoot a 300 RUM without a brake and have loaded for a few and find them finicky so do not want one (personal preference). The 30/375 Ruger should hold ~93 to 94 grains while the 300 win mag is ~87 grains so I'm thinking it will make the difference.
Besides, it's a wildcat, should be fun!