Mac,
Actually you wouldn't be re-inventing the Newton...as you can see...the necked down 375 Ruger case would give you more case capacity...Also...since your necking it down...the neck would grow a bit in length as well...So you would have a very effective cartridge...
When necking down cases to a smaller bore, actually shortens the neck. As the shoulder lengthens to accommodate a smaller bore, it naturally shortens the actual neck. The Ruger 375 case neck is already less length than bore diameter, so reduction in bore diameter to a smaller bore shortens the neck further, and less desirable. The Newton case has a different head size than standard bolt rifles today so wildcatters attempt to use standard existing brass for the designs. With the addition of the 338-06, 338 Federal, .338 Win Mag, 340 Dakota, .40 Weatherby and the 338 RUM, any new wildcat would just duplicate these rounds ballistically. A better round than the 375 Ruger to .338 necked down would be the .333 Rimless Nitro necked up. Minor gunsmithing, equal powder capacity to the .338 Win Mag or slightly more, and a very long neck to hold almost any length bullet while still maintaining overall cartridge length.