I' ve seen this several times, often you cannot see the barrel swell, but can see a disruption in the rifling 2-3" from the muzzle. My best guess on how this happens is that a shooter shots the rifle with anything more than a very fine, thin coating of oil in the barrel and the bullet pushes the thicker layer of oil forward in the barrel like a wiper blade on a windshield until enough builds up at the end of the barrel to cause an overpressure. [] Cut and recrown, and always run a dry patch down the barrel before a shooting session and recheck to make sure the barrrel is clear.