The revolver is a .357 Model 686 with a 4" barrel.
The line doesn't appear smooth like two separate pieces side by side. But looks as if there are many very small tears/rips that combine together to complety circle the inside of the barrel, thus making a perfect ring.
Hard to describe, but if you've ever ground down a bad weld, when you get to the seam you might see a similar appearance.
To answer another question: the rifling is not sharp with well defined edges, but has a smooth appearance.
Almost forgot: It doesn't appear to be a bulge and it is located approx. 1/4" inside the end of the barrel.
What you are describing is a barrel that was machined via a variant of the EDM process. This does NOT produce sharp edged lands and grooves, the lands and grooves are blended together with a radius. As for that mark you are seeing, if it's not causing an accumulation of leading in the area I don't think it should be of any concern. Your 686 uses a one piece barrel and it would take a massive metalurgical defect to the barrel to fail at this point. In addition, it's more likely that what you are seeing could be nothing more than a "stain" due to poor circulation of the electrolight in that particular area, basically it's nothing more that a bit of "etching".
BTW, I have a model 620 that features that "horrible" 2 piece barrel. Last weekend I shot 7 rounds at 35 yards from a sandbag rest after sighting in a new reflex sight and, if you throw out the 2 called flyers when I jerked the trigger, the result was a horizontal line of 5 hits 1.75 inch long. If my trigger technique were a bit better, I have no doubt the gun would have grouped 3/4 inch or less. Ammo was Speer Lawman 125 grain TMJ and apparently the 620 just loves this round. This weekend I plan on buying more and working on my release skills in single action. One day I hope to get good enough to put 7 rounds into a 3/4 inch group with the 620 and you can be sure I'll be doing some bragging on that "horrible" 2 piece barrel.
Note, I've been experimenting with the mainspring tension in my revolvers and using Speer ammunition to insure reliable ignition in double action. Prior to installing the reflex sight on my 620 I tried it with some Blazer 158 grain LRN. I WILL NOT make that mistake again. The leading in the barrel produced by just shooting 50 rounds took a full 3 hours of patching the barrel with a peroxide/vinegar solution to get most of the lead out of the barrel. I also had to resort to using a bronze bore brush on the barrel at the start, something I prefer to avoid using. I STRONGLY ADVISE AGAINST using this particular ammunition in any of the guns that feature the EDM rifling. I am also going to avoid using any non jacketed ammunition in these barrels. It produces a rifling that is somewhat similar in function to the polygonal rifling used in the Glock semi automatics and this type of rifling has been widely reported to have issues with leading when used with non jacketed ammunition. At this point S&W has not advised that only jacketed ammunition be used in these barrel but from my experience I would advise doing so. Bottomline, if you're using commercial ammunition in your 686, stick to the jacketed or semi jacketed ammunition. Handloaders may be able to do some lube/alloy tricks to avoid leading issues but those shooting commercial ammo should take the cautious approach.
As for wearing your barrel out, I don't think you'll live that long. It's my impression that a fired bullet has to exceed at least 2500 fps for there to be enough friction created heat to be generated to cause a high rate of wear. Since anything that is safe to use in a 686 is well below that threshold, it will probably take well in excess off 100,000 rounds to cause any measurable wear. At about 50 cents a round for 357 Magnums, that means you'll have to use about 50 thousand dollars of ammunition to wear out the barrel in your 700 dollar revolver.
