I was shooting a Sig 556xi at a class when it jammed? The Instructor and I pulled this strange looking round out of my 5.56 gun. Just kidding. It looks like the first round fired and for whatever reason the case separated in the chamber leaving about 2/3 rds of the brass. The second round was fed right into the jammed casing. The Sig is a AK style upper piston operated with a lots of stout forward motion of the bolt, enough to place the second round where it now sits. Fortunately not enough to allow me to touch it off. Boy, that would have been fun. So if you'er shooting and it just doesn't feel right stop and inspect. Chris