Tom, looks like one got stuck in the pipe at one time. Fortunately, the low pressure of the 45-70 didn't seem to do much damage. It's a visibly noticeable ring, but with a pick it is hard to tell, so it may be fairly shallow.
Also, with at least 8" of bore left between the ring and the muzzle, I would think that the effect on accuracy would be minimal.
I'll bet a seasoned gunsmith could apply judicious force to mitigate some of the slight enlargement, but if it shoots fine, why bother?