Re: 7mm Rem Mag to .257 caliber?
I don't have any experience with it, but wouldn't it be ballisticaly similar to a 25-06 Imp or 264 Win Mag?
I know the 257 STW was popular at one time.
I know this isn't much help, just added food for thought if you were thinking of building one.
Learn from others mistakes, you won't live long enough to make them all yourself