I reload for the .357 Sig, so will pass along some of my observations. First, resizing die settings are critical for me with RCBS dies. There is a very fine line between not resizing enough so the round will chamber, versus collapsing the shoulder if you go too far. After I load some rounds, I use my pistol's barrel as a check gauge to make sure I'm sizing properly. Also, these are bottleneck cartridges, so have to be lubed.
You will also find that not all 9 mm (.355) bullets will work with the .357 Sig. What you need are bullets with a relatively long bearing surface. If they don't have the long bearing surface, you won't be able to get the short neck area to grip them properly at the relatively short OAL this round needs. Speaking of that, I found I also had to remove the expander ball with my dies, to make sure the case mouth had enough tension to hold bullets well. I just chamfer the case mouth slightly rather than open up with the expander.
This round is fairly high pressure, so I tend to stay with a slower powder. Many people recommend AA #9 or #7. I use #7 for moderate to near-max loads with good success. There is probably more I could add, but can't think of it right now. Good luck with your reloading.
Regards - AZFred