I once had a French MAS 36 rifle converted to .308 Win., which had a tight spot somewhere in the middle of the barrel. Don't have the rifle any more, so I can't re-check, but I believe the tight spot was not near any sight cuts or other features, and I think I remember tool marks at the corresponding point on the outside of the barrel. My guess at the time was that the folks who rechambered it (from 7.5 French to .308 Win.) had probably cranked too hard on the lathe chuck, or whatever they used to hold the barrel, and the steel was soft enough that they crushed it a little. (Barrel steel doesn't seem to be especially hard.)