Stan - I should have swapped out the shell on the spot, but didn't. When the gun seemed to be working again and I took it out for a test fire, I used the five remaining shells that I would have shot if the gun had not malfunctioned. The fail-to-fire shell was among those. When the gun was messing up, it did not sound as though the hammers for either barrel were falling very hard.

I still have the fail-to-fire shell from the second incident and it seems normal in every respect.