The trick will be not damaging the barrel. If it does not come out with some easy prodding with a cleaning rod I would take it to a gunsmith.
If it was mine I have some possible answers in my shop, but most people do not have so many lathes they need a paper and pencil to count them.

On that case that broke it looks like it had some moisture in it the powder. whether it leaked in or the powder broke down into a corrosive liquid, I have no clue. I guess I might keep some to look at and give the rest to a museum?