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The .256 Newton is a rimless, bottleneck case. The rim and head diameter are .468-470, the same as the .30-'06. Case length is 2.457, outside neck diameter is .289. It looks a little like a sharp shoulder 7 x 57.
The .256 Newton can be made from .30-'06 by trimming the case to 2.457, running the case into a .257 Roberts die until the rifle bolt will close on the case, then expand the neck to an inside diameter of .264; neck ream if necessary. Use .264 bullets. Fire form with light loads.
Once cases are fireformed, loading data for the 6.5 x 55 should be about right; as always, start low and work up. You should be able to neck size in a 6.5 x 55 die.
I don't know if .256 Newton dies are available, but you might try RCBS, they have a lot of odd-wad dies. If you can send them a fired, fireformed case, they may be able to help.
Jim
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