Rebore sounds like a great idea, but you need to check muzzle diameter to see how big you can go. I was going to do a rebore and rechamber to one of the 375 Ruger based cases with a similar 77 with a magnum chamber. It was a few years ago, as I recall, there is a little bit of the chamber that is bigger on the belted magnums so that it wasn't as simple as running a chambering reamer in and opening up the chamber. I would have needed to set back the barrel then rebore, rechamber. I dropped that project when I considered that I would end up with a pretty lightweight barrel and not exactly the cartridge I wanted and way more cost than I wanted to put in it.
I did end up with a rebarrel to 416 Talor which I loved. It might be a little powerful for your needs, but it sure was fun.
Good luck,
Bfly
