Unburned powder is what we see in sparks exiting the muzzle & gap between the cylinder & barrel, for the most part. In guns that have no gap, unburned black powder will ignite as it exits the barrel with no damage to the gun. There are limits, of course, but as long as the projectile/shot load exit, the barrel will most likely stay intact.
Patches & wads might ignite, too, which adds to the pyrotechnic footprint. In my experience, pyrodex pellets sometimes don't fully burn inside a barrel, so you can see them flying downrange following the projectile for a bit. It's all great fun ..if you don't set the woods ablaze.
