You can use a 300 with that twist, not sure where you'll find the case capacity to push it though.

so i take it loading up with 110 or 125 grain bullets would be to light?? i dont really underatand how all the twist and bullet weight work with each other.

If I were going to the trouble to rebarrel a lever action 357, I'd want a 1 in 20" twist, if I could find one. A 1-20 will stabilize up to 200 grains. Excessive spin is not helpful to cast bullets.

Also, I'd cut the throat with a custom reamer instead of the SAAMI "toilet bowl" throat. If you're going to spend the money on a custom barrel, why not do it right?

The rule of thumb is, the heavier the bullet the faster the twist. I believe that it is actually the longer the bullet the faster the twist. (somebody can correct me if I'm wrong about the weight). SOOO if you are shooting a 25 cal with a 1 in 10 twist it will be most accurate with a heavier bullet then a 1 in 14 twist which would be most accurate with a lighter bullet. It all has to do with the stabilization of the bullet.
Mike'