In this case I think the cap jams are related to the cylinders I was using. I did a trade for some used Remington cylinders. I thought I'd save time on range day by pre-loading a bunch of cylinders at home the night before.
All my jams were on those spare cylinders. The cylinders supplied with the guns worked fine.
The cylinders I got in trade were advertised as Pietta cylinders, and they all mounted on my gun, so I thought I was good to go. But I was wrong
All those spare cylinders were a bit short in my Pietta NMAs. There was enough fore and aft movement under the impact of the hammer fall and from recoil to cause the jams.
As it turns out, those cylinders fit my Uberti NMA perfectly.
Lesson learned...Uberti cylinders, being smaller will function on a Pietta gun, but you can't get a Pietta cylinder into an Uberti gun...