That could be a build up of burnt in varnish there also. I would take a jag, a wad of 0000 steel wook, get it in the barrel & squirt a lil kerocene in there & scrub the bejesus at that area. That may come off or out & you are not going to damage the barrel with the 0000 steel wool. Pull it out after about 50 back & forth scrubs, refluf the steel wool & do it again. Now keep in mine you have to have this steel wool tight on the jag & I usually use a jag 1 caliber under the bore. (on a .54 I use a .50 jag) this way you have some room for the steel wood, but you want it in there really Tight.
Put a wad of steel wood at the bore, take the jag on a cleaning rod & go in a small circle with pressure on the jag & force it in, this will make it tight but you need that tight fit. If it goes in esily it will not do any good.
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