I was in Cabelas Saturday and couldn't resist. The ATF can't decide on these now that they have approved them. They initially said it was ok to shoulder them but they have since rescinded that determination. Now if you shoulder it you have redesigned(their words) and changed the manufacturers intent, it would be a SBR and fall under the NFA.
This came as a kit in 300 BLK and 5.56mm with an additional conversion in 7.62x39mm available at a later date. It is a new design with a new gas system, totally modular and ambidextrous controls. I can't wait to try it out. I will probably add a suppressor and will register it as a SBR with a different stock arrangement.
http://www.sigevolution.com/sigmcx