Having worked my entire career around different steel alloys I've never heard of "undistributed alloys in material". Inculsions definitely. These inclusions are usually slag or some other foreign material that got caught in the pour of the ingot, forging or casting. Other defects include porosity, fissures, cracks, etc.
This is the reason most finished pieces are checked by some form of non-destructive testing like ultrasonic or magnetic particle or even X-ray testing.
Another thing to keep in mind is that muzzleloader barrels do not go through the same testing as barrels that use smokeless powders. Not even test firing (This really scares the crap out of me).
I assume the reason for this is because black powder or the substitutes burn at the same rate when in the open or confined (like in a barrel) for an even pressure throughout the burn. Smokeless powder when confined continues to build up pressure as it burns until it is unconfined or burned up. Also black powder is defined as an explosive but smokeless powder is defined as a propellant.