Congradulations on your good fortune, that nobody was injured, or worse.
FWIW, I wouldn't recommend firing or working on ANY old/strange gun, without FIRST dropping the ramrod downbore, marking it at the muzzle, then withdrawing it and holding it alongside the barrel to the mark, to see if it's loaded.
Even if it WAS loaded, I
still wouldn't fire it, even outside - since the charge is unknown, and may be a double load or even smokeless powder.
I would mechanically pull the bullet from the muzzle, or blow it out from the rear with compressed air, then dig out whatever powder was in there with a worm or jag.
.