Observations:
1. The barrel is old.
2 Barrel is from a rocklock.
3 There is more crud than rust.
4. Some rifling is visible.
5. We don'tknow what the bore looked like.
6. Proper cleaning would have prevented this.
Be very careful with muzzleloader guns made before 1900. The iron and steel used in those barrels is mostly of poor quality. This is especially true of barrels made of wrought iron: They often contain inclusions and slag that promote rusting.
In about 1970 i was present at a blackpowder shooting match when a 100 plus year old gun burst, blowing off about one foot of the barrel. The gun was in pristine shape and had been well cared for. The shooter was not injured. That barrel was made of the sorriest stuff i ever seen.