[j357]

I loaded up some light .357 Magnums a few months back and had a chance to shoot them today. Near the end of the box I had one set of six cases that did not easily drop from the cylinder. Found a split case that concerns me. I suspect it was a bad case, I just can't figure out what happened outside that.
Details:
Mixed .357 head stamp all nickle plated. Split case is a Federal, and had 6 more in the box that show no problems. 125 grain Semi-jacketed FP, CCI 500 primers, loaded with 6.6 grains of Clays Universal - Lee powder measure on at LCT press, firm crimp on the groove. All shot in a SW Mod 28 which shows no sign of problems or damage. Subject round produced no noticeable difference in recoil. Pictures attached.

Any insight on what could have caused this would be appreciated.

EDIT: 6.6 grains Universal is well below the general minimum load I could find from several sources = 7.0 was the starting load number I located, but figured I would even drop back from that.