I am working the kinks out of rechambering a 300 win mag into a 30/375 Ruger. The reamer can be rented from 4D
The case capacity is 93.6 grs as compared to the 300 win mag at around 87 grs. Should still be able to shoot without a muzzle brake.
Now the 338/375 Ruger should also be interesting and be somewhere between the 338 win mag and the 338RUM but would be a recoil bad boy from the bench and personally I would need a brake.
To resize the case I got a 338RCM Full Length Die and a 300RCM neck sizer and I can get the brass neck down to 30 caliber
30/375 Ruger..................375 Ruger
It makes a good looking case
300 win mag...................30/375 Ruger.........................375 Ruger
Now in my case with a rechamber of an existing 30 caliber, I will only have to set the barrel back a couple of threads. But in your case with a total rebarrel you will only need to order a 30 caliber barrel from whichever barrel maker you want and have the chamber reamed to what you want.
James, what kind of performance are you getting with your 30 caliber?