This could be riveling. It has been conjectured that this wrinkling of a bore is caused by a certain often repeated harmonic vibration.
This is probably true. If so, it may be the first symptom of a soon to fail barrel. Considering the possible cause of the recent bridge collapse, we can see that even modern steel is subject to fatigue and can fail.
These "wrinkles" can be honed out and leave the bore smooth but still compromised.
I would suggest an examination , by a competent smith, before shooting any riveled gun.
Best,
John
