If you have a set a head space gauges,and a pull thru reamer......remember to check head space, with out extractor, firepin,ect......personally I'd keep it 357 and shoot it......
Aaron
+1
I have done similar things making some ackley improved wildcats. IF properly set up, it is not too tricky of a job. Getting properly set up will likely be as expensive as just having a gunsmith do it for you, which will not be cheap. (who has a roughing and a finishing rifle reamer set up for that caliber just sitting on the shelf?)
I am assuming your old Martini was a .310 cadet. I like the idea of adding another 200 plus fps to the velocity in a rifle length .357. Are you loading your own with very slow powder, or shooting factory? I have seen an article where .357 ammo designed for a handgun, gains some with the carbine/rifle barrel lengths, but really comes into performance with powders designed for longer barrels. Custom ammo might get you part way there without buying reamers.
If rechambering isn't a viable option, I don't want it.
It's a viable option. Just depends on how much you are prepared to spend to do it.