What are you necking down and to what?

I am just trying to get a general idea of the process of necking down one caliber to a different one. I'm  wondering what a .43 Spanish ( Remington ) carbine  could be made from.
Thanks 

Blacki

Actually the .43 Spanish can be made from .348 Win brass, according toThe Home Guide to Cartridge Conversions by George Nonte. It needs to be opened up rather than necked down. The .348 is another problem, as it's hard to come by also. You can get brass for either at Buffalo Arms, or Rocky Mountain Cartridge Co. They both sell .43 Spanish cases.

If it is a carbine, a chamber cast may be in order, especially before buying dies, etc.  In the military black powder cartridge days, there were quite often carbine cartridges that were short versions.  There was often a one-way interchangeability.  You could shoot the carbine cartridges in the rifle, but the rifle cartridges would not chamber in the carbine.   

I am a cartridge collector, and there are 11 mm Spanish carbine, the standard 11 mm Spanish bottlenecked rifle case, and a latter day version called the 11.15 mm Reformado on a lot of lists.  The Reformado is a larger tapered case version.  The Spanish carbine cartridge is a lot pricier than the other two.  I think there are rolling block carbines chambered for all 3 cartridges, depending on when & where they were made. ???