I can't help noticing that the photos of ring bulge damage attributed to gas hammer doesn't even remotely resemble rivelling.
I'd also think that the risks associated with propellant fracturing wouldn't be as likely to occur in a shotgun shell because the powder charge is held firmly in place by the wads, shot, and crimp. The cases I've read bout with rifle ammunition have resulted when cartridges rolled around and were subjected to prolonged vibration as when laying on the dashboard of a pick-up truck for many miles on rough roads. This probably wouldn't be much of a problem in rifle or handgun ammunition that had compressed powder charges.
