BGlass,
The decision of what donor case to use to make obsolete cartridge cases is almost always one to either use a case larger and swage the head smaller or use one smaller and fireform it ahead of the head (of course necessary modification of case length and rim diameter are also considerations). When evaluating donor cases based on a chamber cast, the finished cartridge must always be smaller than the chamber, therefore smaller than the chamber cast. It is easier to expand a donor case by fireforming (within reason) than it is to swage the head smaller. The question of how much smaller than the chamber should the cartridge be is not precise, but a general rule of thumb is from .005" to .015", when possible, with the caveat that a little more may be acceptable if the finished cartridge is not a high pressure one. Based on this, I suggest you take a look at 348 Winchester or 450-400 3". Neither of these are easily available but 348 (or the wildcat 450 Alaskan) may be available on a seasonal basis from Sterline and 450-400 3" cases are made by Hornady on a limited basis. Both of these are a little expensive, but you won't need many.
Mike
