Most barrel bursts are due to an obstruction in the bore. A bulge may also be caused by an obstruction that is not enough to actually burst the barrel, such as a light plug of snow in the bore.
While dents and bulges are both a displacement of metal, a dent may be worse than a bulge of similar displacement because it partially obstructs the bore. Also, a dent may be a shallow depression, or it may have been caused by a sharp object that leaves a more stress damaged area.
If the bulge you describe on your Fox is so minor that you have to hold the barrels up to light to even see it, and you can't really feel it, then I wouldn't lose any sleep over it. In fact, attempting to hammer it down could pop a rib loose.
In fact, something that minor might be a short section of rivelling. Rivelling is a weird phenomena we sometimes see that typically occurs in areas where the barrel wall thickness is thinner. It may occur in a very short section, or run for half the length of the barrel. It can look like one or more small ring bulges, or a series of ripples, and is often barely visible unless you hold the barrels at the right angle to a light source. There are several theories as to what causes it, from shot bridging, to gas hammer, to a partial blockage of the bore caused by a shot-filled wad trying to displace excessive oil as it moves down the bore. I personally think the excessive oil in the bore theory is the most plausible considering the hydraulic forces that might be generated in that scenario. There is a pretty good discussion about rivelling in "Shotgun Technicana". I have never heard of any problems that arose strictly from firing a gun that has rivelling, and wouldn't hesitate myself unless barrel wall thickness in that area was marginal.
