I bought a barrel and recoil spring to convert a SIG P239 from .40SW to 257SIG years ago. Now I only see kits for the P320 and P229. I wasn't happy with the results, in terms of ballistics and ammunition cost, and never attempted to reload 357 SIG.
If you want .357 Magnum ballistics, look to 10mm. 357 SIG promised but didn't deliver, but 10mm has enjoyed a dramatic recovery in popularity. Consider 10mm to be .40SW on steroids (having once suffered the reverse).
