Kevin W. We had a really long discussion on this maybe a year ago and it is probably in the archives. Some really good stuff came out of that talk. To review, it was found that in MOST cases the chain firing occurred on the first batch of the day shots, in other words, you load up an empty gun, fire the first six rounds and that's when a lot of the chain firing occurred. It was less common after a few cylinders of rounds had been fired.
One guy loaded powder and wads, no bullets, and QUICKLY passed a propane torch over uncapped nipples, trying to create the same situation as hot gases over a nipple where the cap fell off, the cylinders ignited on that fast pass- so it is sure possible for that to cause a chain firing.
Another fellow thought perhaps the recoil moved the balls in some of the other chambers enough to invite hot gases- sure another possibility.
It was also found that lube placed over the balls quickly melted away after a shot or two. In other words if you load powder, ball, crisco, fire three rounds and then look at the end of the still unfired chambers, many times the lube has burned/melted off- not offering any protection. Historically, I think the lube had wax. etc that gave better preformance.
I any event, the general thought- as I recall- was to always fire off caps on the nipples prior to loading to burn off any grease/oil, perhaps the grease/oil caused the caps to fall off. After that load with powder, a lubed wad, and ball. This gave better protection and make sure there is a ring of lead, insuring a tight fit on the ball.
If the caps don't fit properly on the nipples, use different caps or get a new set of nipples.
Wads are PC, Sam Colt had them on his patent for combustible cartridges.