I got this from another forum - but it works.
Use a .40 S&W carbide die to resize the .357 SIG brass first.
The run it through a standard .357 SIG die - without sizing lube. The neck and shoulder are such a small area that as long as the body is already resized the std. .357 SIG resizing die will take care of those areas just fine/easy.
After that, it's just belling the case mouth to accept the new bullet, putting the primers in, adding powder, seating the bullet and crimping - all done the same way as any straight walled case.
I did run into an issue earlier this year (or was it late last year??) where I was tearing up some of the cases. I'd get that dreaded bulge right at the transition from case side wall to shoulder due to some brass being longer than others and then the die bulged the long cases when crimping them.
All I was doing was removing the bell/flare on the case mouth but the longer cases where getting really pressured by that step in the die that crimps/removes the bell in the case mouth.
Neither my Glock M31 or my full sized M&P are much for shooting good groups and I stopped shooting them when the CZ's spoiled me with their accuracy (9MM and .40 S&W).
I like the caliber and it's potential, I just want a pistol that shoots good groups. Be sure, Timetofly, to come back and show us some groups.