A bulge almost always affects the area between the ribs as well. I doubt there are many repairs of bulges where the gunsmith strips the ribs off and does any cold forming of the area between the ribs. So, a bulge will likely show as a 'wave' in the bore in the area of the ribs. It will be a subtle change in the light reflection in the bore. It might be difficult to see, but should be visible. I've seen it even on subtle bulges.
I would not be overly concerned with small bulges, especially out in the thin section of the barrel past the forend. But a large bulge, especially as it gets in around the forend is a big concern to me and I would pass on a gun that had one or be thinking sleeving it when assessing whether or not to buy it.
Any cold working of the steel affects the properties. Dents and bulges alike. For the most part, they probably do more to strengthen the area than reduce strength. A review of a typical stress/strain tensile test chart of a steel in the range of hardness of barrels would show you this. Just after the point at which the force required to yield (permanently deform) the coupon, the force required to yield it further is less. Then, the force required to yield it further goes up well beyond the original yield point. This is a result of "work hardening" the steel. As the grain structure is changed from the "work hardening", it becomes stronger until it finally fails. This is also why hammer forged barrels are very strong. Of course, "work hardening" can be over done and lead to earlier failure as well. Some of the barrel failures of the stainless Sako bolt guns may have been an example of this. For us with ole shotguns, I wouldn't worry about work hardening as a problem. Rather wall thickness and crack detection would be comforting to me. Your comfort requirements may vary.
