Re: 338 ultra improved???
As far as I know is that 338 Tomahawk is based on 300 ULTRA RUM, blown out the shoulder (a little wider and 35 deg)and necked up to .338 cal.
I'm not sure what to call it for 338 Ultra Imp. (Perhaps call it 338 Edge). This brass is a little shorter than 300 Ultra RUM. If you are looking for more capacity of powder, go for 300 RUM brass or 375 brass and neck down to 338.
Does this make sense to you?