For more information go to
www.superiorballistics.com. The SMc cartridges were developed by a ballistics and handloading expert, and a rocket motor design engineer. The idea was to develop a cartridge that maximized "efficiency"; that is, got the most velocity per grain of powder used. They can explain it on their website better than I can.
The 5 X 35mm SMc is, I think, the only commercially available round. Savage's custom shop can chamber a rifle for it and I think brass is available. Pretty interesting stuff, in my opinion. I hope they have success and are able to introduce additional cartridges.