I've got an old (1961) cartridge conversion book that recommends using 30/40 brass,


" turn rim down to .505" diameter; Swage head to .442" diameter; Trim to 2.01 length; Size full length; Use .308" bullets. " from 'Cartridge Conversions' by Nonte.

Don't know much about the book or author, but it came out of old-school gunsmith's shop from the '50's thru 70's.