The anvil of a hydraulic dent raiser would only exert force upon a small portion of the circumference. But you might consider the possibility of a different type of hydraulic forces at play. I'll toss out the THEORY that this damage could be the result of an excessive amount of grease or heavy oil in the bore when it was fired. I see a less prominent annular ring bulge beyond the two larger rings, so this might be an extreme example of rivveling.

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug