The .375 Ruger appears to be based on a shortened, necked down .404, very similar and in between the short and ultra mags. It's base diameter is .532" (same diameter as the belt on a standard belted magnum case) and it's 2.5" long. Check your J.J. Donnely's book or CTG. of the World for similar case measurements. The improtant measurement is the base diameter. Shorter cases are simple to make from any length brass. The base diameter is the clincher.