As I learned elsewhere. It is a good idea to check any or old muzzleloaders to see if some old charge is still in the barrels. Measure a rod on the outside of the barrels. See how far the road will go inside the barrel. Any significant difference could be a load of old powder, shot or ball etc.

Having said that, I still have my great grandfathers old rusted side by side ML shotgun and have never checked it!