Can you find a gunsmith or someone else with a bore scope so you can actually look that the rough spot? If you can see into the bore, can you spot any roughness?
When you last fired the rifle, did you notice any problems?
It could be rust if the bore got wet or the gun was stored in a moist area. Of it could be fouling if you have been firing cast bullet loads. That is too far forward to be throat erosion, plus I doubt you have fired the rifle enough to worry about throat erosion.