Interesting piece .
I think you were on the right track when you started out bill .
The idea of superimposing charges to effect repeated firing wasn’t new .
There are 18th century flintlocks that do just that but have 4 or 5 super imposed charges .
The biggest reason I think they never really caught on is that they were or at least appear to be a little complicated to shoot and load . In some cases probably mechanically challenged as well . Not to also mention the possibility that someone could set of the rear charge prior to setting off the front charge .
Which IMO would be a far larger concern then a chain fire