One ignorant person left a comment at that YouTube video's comment section, saying: "Chainfires are a myth".
Well if they are a myth, then I guess I was involved with a myth the very first time and very first shot I ever fired from a cap and ball revolver. The very first cap and ball revolver I owned was a Walker I bought back in the early '70's. Didn't know much about how to load back then, so I just filled two chambers up with powder, levered the balls in, capped the nipples and fired. It fired the chamber under the hammer and chainfired the chamber next to it. That was a long time ago, and I've learned a little since then lol. Never had a chain fire since then.
But yeah, chainfiring is just a myth, uh huh
Good video, nicely showing how a chainfire can occur.