Well, I am working from memory so cut me some lack but as I recall the last time I had this discussion one guy used a propane torch and put powder and wads in all the chambers but no caps on the nipples. A quick wrist flip of the torch over the nipples blew all the chambers but the wads didn't result in any firings. Several things were deduced.
He must have assumed that a chain fire is caused by fire going directly into the nipples. I think the loose caps are ignited by flame from firing, then they pop and that is what sets off the adjacent charge.