I load 357sig on a Dillon 550B. The carbide dies are the only way to go IOhttp://www.tjconevera.com/berrys-bullets.htmlMO, & worth the cost. I don't know about trimming, I haven't gone thru the first 1000 cases yet, but have reloaded a small batch several times to check longevity & haven't need to trim yet after 4 firings, I doubt it will be required.
The biggest problem of the 357sig is bullet setback. You can NOT use any 9mm bullet & get good results. You MUST use a tuncated cone profile, JHP, FMJ or Lead, but the closer the profile is to a TC, the better neck tension you get. Set the dies up correctly to size the case so it headspaces on the little shoulder, a tiny bit of flare, a good taper crimp & it's not really diff. than loading anything else. The 357sig is what the 9mm+P+ wishes it was.
