I have no idea how long ago now, but do recall reading a discussion of this very thing in the American Rifleman years ago, back when they still had much useful info in their publication. The consensus was that as smokeless powder deteriated a decrease in pressures would be present, never an increase. What generally, but not always, occurs in old ammo is a failure of the priming compound prior to an actual failure of the powder. Sounds as if Joes ammo may have been an exception, or perhaps a weak primer was just not giving good ignition & the powder didn't ignite fast enough to develope good pressure. It would thus seem the major concern in firing old ammo is to be sure everything "Clears" the bbl, not leaving behind an obstruction.
