1881 Scott Dimple:

[Aaron Beck]

I also assumed an errant shot, I have a similar set of barrels but decided to let sleeping dogs lie, especially with a composite steel barrel.

[Drew Hause]

Aaron may be correct. 8" from the muzzle the pressure is <1000 psi and likely less than 500 psi if Bruce is using low pressure loads.
Blowing up the image, the defect is immediately adjacent to the riband end weld and I would be very apprehensive about the use of a hydraulic dent lifter.