The new, newer, newest .375 Ruger is said to be on the same head diameter as the .30 Newton. Sure looks like you could neck down that brass, when actually on the market. I bet some enterprising custom riflemaker will do that and advertise The Return of the Newton. More useful than a .375, anyhow.
Huntington and Buffalo Arms list .30 Newton brass from Jamison,
http://www.huntingtons.com/cases_jamison.html
http://www.buffaloarms.com/browse.cf...atid=35&step=2
The .30 Belted Newton aka .30-338 has nothing to do with Charles Newton except to give about the same ballistics as the .30 Newton with common brass.
Be careful with Sharpe's load data. Powders have changed in the past 50+ years and his loads might not be what they appear.