Forget about it. Go with a 9mm Commander and have it fitted with a 38 Superb barrel.
If you really actually have to go to 357 Sig then you need to start with a .40 SW or 10 mm frame and slide in order to get it to function properly. ie: magazine feeding, ejecting, and extraction.
Just not worth it in my mind. YMMV
I have one of those Sig "almost a 1911" Nightmares in 357 and really enjoy it. I reload for the 357, and the long action of the 1911 gives me options that short action pistols don't. I can load to much greater than normal OAL for the 357, which allows me to use pretty much any weight/shape 9mm bullet out there. Runs great with standard 10mm magazines. Once you have brass, reloading the 357 is as cheap as loading 9mm. But you can play with rounds that run from 9mm velocities up through the Sig "magnum" ones. If you're a reloader and a tinkerer, go for it!
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