4D makes a reamer that a smith can rent for a 30/375 Ruger
4-D Products Rentals
you gain about 6 or 7 grains over the 300 win mag but in an unbelted case. I have been planning one and have necked down a 375 Ruger case just to make sure I can get it going to get a fire formed case to send off to have a die made. I talked with Hornady and one of their techs shoot one so he clued me in on the process. It is too much of a step down to go from 375 to 308 in one step. So this is the process I worked out:
1. Size the neck as small as possible with a 375 Ruger Full Length die with the expander removed
2. Use a 338RCM Full length die (a neck die won't work because the forcing cone will crumple the neck). A Full length die has a more gradual forcing cone evidently
3. Use a 300 RCM Neck Sizer to size the neck down to 308
Here is a comparison between the 300 win mag, 30/375 Ruger and 375 Ruger cases
My plan is to send off a 300 win mag rifle and have the chamber reamed out. The smith may have to set it back a couple of threads but should not have to refinish the gun so it should be economical. Just haven't gotten around to it yet.
My goal is to shoot a 200 gr 308 bullet at an honest 3000 fps without a belt and without a muzzle brake. And just because it will be different and fun.