Its bascially a 9mm on steroids and shoots a .355" or 9MM bullet. Think they named it 357sig for marketing. Bascially a 40S&W necked down to .355. But you can't use 40S&W case due to the necking down makes them too short. You can get it up to terrific speeds.

Pros:
Feeds nice due to bottleneck
Very fast for a 9mm bullet. Well over supersonic

Cons:
Very difficult to reload. I got one, and I don't even bother anymore. I use my 40Barrel. Reason so hard to reload is the bottleneck design makes very low neck tension. So easy to push back in the brass. Only way to get around is with a 40S&W lee factory crimp with a 9mm insert. Works ok. Still not worth my trouble.

Looks like its dieing quickly. Ammo is getting harder to find and expensive. So I bought up 1000 pieces of brass.

Brought out to simulate 125gr performance of a 357Mag wheelgun. Still don't understand that one. You can load a 357mag up to 1600fps with a 125gr.

Stick with 40cal. Great balance of speed, bullets, recoil, and availability. Much more powerful than a 9mm. About the same as a 357Sig.