Lucky and inattentive, it does look like the initial cut was done on a mill, and then a belt sander, the deep cuts are fairly straight, but the grain is all one directional instead of circular, and some of the edges are slightly rounded + the burr.
Had this happen to myself, shooting light .38 loads in a plate match, a really fast stage with big targets, 18x24, up close, probably 5 shots in 2 seconds to 2.5 seconds, from the holster, had a squib, but the booger hook was on the bang switch going for the gold, 3rd target no hit, 4th target 2 hits. It did bulge the barrel right in the forcing cone area, the old 686 shoots just as good as it ever did, you can only tell when you run a cleaning brush down her, it gets real easy for a 1/4". No RO would have been fast enough in this instance, .15 second splits. Like M58 said.
Last summer, the match director for the steel match, called me out and asked if I could help on a stuck 1911. We went to our tool shed, which is not bad, and there was a shooter with a Kimber that the slide would not operate, and the mention of squibs came out, it took me 20 MINUTES TO GET THIS APART. I had to use a vise and lube to get the barrel bushing out, after that I worked the barrel back and forth, and finally was able to remove the barrel, it looked like a reamer, it split at every groove, and was bulged big time, there were 2 bullets stuck, a squib sent one far enough to get the bullet out of the way of the next round, but the RO did not stop him, and he chambered the next in a clearance drill, and I expect the next was an underload as well, as it did not blow the grips off, but definitely locked up the pistol. I do need a pic as once it was apart, it was damn near comical.