I picked up a couple of odd looking shotshells at SLICS and for awhile they had me baffled (2 on left. 1 on right is 16ga for rim comparrison). They had no gauge on them, though they appeared to be 16ga, and the rim was noticeably larger, fatter than normal. Though the cases were marked XPERT and MARK 5 I knew they were experimental steel shot loads.
Bob Reichenbach solved the mystery for me. The two shells on the left are for the bench gun used in the Olin lab to test shells that were to potent for standard guns. The thicker rim would make them impossible to fit in a standard chamber. I have quite a number of experimental shells, particularly with steel shot loads, but these are the only ones with the heavy rim I have come across.