I used to shoot percussion revolvers quite often back in the 60's and 70's. I've only had one chain fire and that was with a Ruger Old Army using Crisco over the balls on a hot day. I don't know if I just didn't get enough of the grease where it belonged or if the heat made the grease run out, but it fired the chamber in line with the barrel and the ones on each side of the barrel. Three shots at once using full powder charges! I thank God that Bill Ruger built some nice strong revolvers!
There was no damage to the gun from this happening and the only wounds I received were a sliver or two of lead and a thoroughly deflated ergo. Bob