Am I to understand that you took a perfectly good Winchester M92 like maybe in .25-20 and made it into a .256 Win. Mag? Gramps gave me an octagon barreled M92 .25-20 and having grown up when the Ruger Hawkeye and the .256 Win. Mag hit the market, the thought did cross my mind to change the .25-20 into a .256 WM. Hard as .25-20 factory ammo is to find, .256 WM would be impossible and that old buck horn sight arrangement doesn't lend itself to magnum shooting distances anyway.
