Thanks for posting this story..not very often that this is found / reported.
I too am curious as to chambering of rifle,( caliber) as well as other pertinent information..
How old is rifle ? any idea of # of rds. fired, before finding the defect ?
How / why did you find the defect ? Something happen that made you grab a bore scooe ?
Factory loads or hand loads used ?
I doubt that any ammo would result in the observed damage.
My guess it's a material defect of some sort.
Nice that the barrel ? rifle ? is going to be replaced.
Please post some follow up info as it becomes available.