dent in barrel
This sounds like a barrel bulge from firing a round down an obstructed barrel. A good gunsmith can remove most of the bulge, and the gun should be ok to shoot without worry of a burst barrel. I have a Miroku O/U with a similar condition that was caused by a previous owner, and I shoot it. I just use standard pressure loads in that barrel. I haven't patterned it but that may be the worst thing about it.