It sounds like a rust "ring " in the barrel, where someone left a damp PRB in the barrel, that rusted the barrel at that point. It also could be a bulge, but I doubt it. Work at it with a bore brush, and steel wool to see if you can't polish out the " bump". Sometimes, just leaving the barrel stand upright, with the nipple plugged, with the barrel full of water and soap, will dissolve these kinds of "rings". This is what occurs frequently when Pyrodex is used, BTW, instead of black powder. While I don't have any first hand experience with other substitute powders, reports here on the forum indicate other substitute powders also develop a "crud ring " in the barrel when used, and the barrel is not scrubbed clean quickly after use. I have used Pearl Drops Toothpaste on damp patches with my jag, to polish a rough spot in the bottom of my barrel on my .50, and over the years, it has slowly become smoother. I know it works, because the patches come out BLACK from my scrubbing. A small square of the 3M Green "scrubbies" can also be used with a jag to attack such crud rings. Some Bartender's Helper- a non-abrasive cleaner--- on the green scrubbie can be put to good use, too.
