Sloppy machining. I think he had the lathe offset to an 11 degree cross feed. The saddle is mostly stationary and the crossfeed is used to introduce the cutting bit to the object to be machined. The machinist went it too deep and missed the surface, thus creating the ring. I've done that myself (but not on a barrel).
A special crowning bit is ground by the barrel maker. It is fed in straight (as opposed an ordinary cutting bit that is fed in from the side as the barrel is rotated) into the muzzle.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!