[Gatofeo]

I experienced three separate occasions of multiple ignition ("chain fire") with the same revolver in the 1970s.
Trying to recall the exact order, and I've posted this so many times before, that my recall of which chambers went may be incorrect, but the incidents are real enough.
First time, chamber to the right of the barrel (2 o'clock) went off. No damage.
Second time, same chamber and the bottom chamber (6 o'clock) went off. Ball lodged in rammer but I pried it out with pocket knife.
Third time, ball to left of barrel (10 o'clock) and 6 o'clock chamber went. Another ball lodged in rammer, bending it.
It was a cheap, brass-framed 1851 Navy, probably made by Italian apprentices on a Monday at noon, so I junked the gun for parts.
In all instances, I use a .451 inch ball with Crisco smeared over it.
The caps were not pinched onto the nipples. That's a practice I later adopted.
I'm not a believer that mutiple ignitions occur from the front, with flame or hot gases getting around the ball. I believe that a missing or loose cap is to blame. Either the cap falls off unnoticed during handling, or is knocked off from recoil.
Good nipples, with caps that fit snug on them, is crucial. I still pinch the caps into an oval shape, just to ensure they cling a little better.
I have not had a multiple ignition since.

Enjoyed the story about the hood of your new, white Chevy getting scorched.
I suffered no injuries from my experiences, but I sure knew something was wrong by the increased recoil and noise.
All chambers went at once, there was no sensation of a gap between their firing. For a brief second, until I saw the cylinder was still intact, I thought the whole shebang had blown sky-high.
Been wearing eye and ear protection since!