15" sounds about where the barrel taper straightens out. The typical shotgun barrel has two tapers that are blended as best as possible by hand.
When metal is cold formed, it hardens and toughens up a bit. A bulge or dent is cold forming. The more working, the more hardening and toughening...to a point.
Hard steels cannot be easily formed or may not yield (form) without fracturing at all if exceptionally hard. Nothing ground breaking here, I have a tremendous grasp of the obvious.
Formable steels have limits of how much they can stretch. The softer the steel, generally, the more it will stretch. But, there are alloys that stretch more than others of similar hardness.
Generally, shotgun barrel steels are relatively soft and form easily. That's why they bulge and dent readily. That's why then can be fixed as well.
The ultimate "bulge" reduction in a barrel is a rotary hamberforged barrel. It starts as a short large diameter bar with a large hole and is literally hammered until it shrinks in diameter and grows in length. Tony G showed me his rotary forge machine that he hadn't implemented yet, but planned to.
A bulge should be assessed by a knowledgeable gunsmith. Frankly, I don't know how they gain the knowledge other than someone taught them and so on, but someone undoubtedly fixed dents and they didn't or did fail and they learned from that and passed it on.
But, one thing that is not so subjective is doing a crack inspection. That can be done visually with strong magnification or with enhanced NDI methods.
Frankly, from your discription of "slight bulge" I would say the metal will be plenty strong after it is formed back or reduced. That's not to say it can be fixed, given there is a rib on top and another barrel below.
I'd give a trusted gunsmith a look at the barrels for an assessment.
