Do not leave gunpowder in your firearm.
There is more than one manufacturer of gunpowder, and at least one who has changed details of mfg over the years.
Somewhere I have notes on one Mad Max examined with a hole eaten through the barrel, from havingbeen left loaded with one maker's early product. Black powder in the barrel MAY be relatively harmless (if you are fool enough to consider a loaded gun "harmless"), but it may also be most destructive.