I have no idea why the bulge-O is correlates with Ken's sabot, but for JoE's edification, I'll say that old G. T. Garwood had a theory about the "juddering" which produces "riveling" (a series of slight bulges which are not prominent as is an obstruction bulge or bridging bulge at the choke but can be detected by the simple expedient of removing bluing from the barrels with a flat sanding block which bridges the depressions between bulges and removes blue from the crest of each rivel. I see no reason why cleaning fluid or oil might not create a pressure excursion just in advance of the shot charge and a resultant nearly undetectable ring bulge.
jack
