Kart barrel stain/fouling

I have a new to me Les Baer wad gun. Inside the end of the barrel there is a stain/ dirty spot that has resisted my attempts to clean it. First, I would like to be the like the medical profession: and do no harm.

I have tried Hoppes on a brass brush, Butches bore shine, JB bore cleaner, shooters choice. All on a patch or at most a brass brush. I want to go in with a sharp instrument and try to scrape it clean... but fear I will ruin the barrel.

Anyone have a suggestion- or should I just leave it be

Thanks

Obviously a question that should be asked of Les.
Regardless, either lead deposits or corrosion can’t be acceptable

From the picture, it looks like exfoliation corrosion.  No fix except a new barrel at this point.

I don't know the type of fouling shown, but had horrible lead fouling on Dan Wesson from one sh!t brand of ammo.  No matter what I did it would not leave. So Hoppes 9 Copper and lead remover, put on paper towel rolled up to fit the barrel, oversoak the paper towel and put in the barrel. Place in ziplock, rotate every few hours for about 12 hours. Remove, put several soaked patches through, rinse with water to remove chemicals, dry. Put wire brush through and a drop of oil. I actually had to do this few times, that's how clingy that crap was. I thought I for sure screwed the barrel, but no, all good )))

A long time ago a friend and I got our brand new Pardini SP Rapid fire pistols with very similar marks on the inside of the barrels.
In our case it turned out to be water based cutting fluid left in the barrels during production that caused corrosion spots.
Apparently they had switched to new cutting fluids and being unaware of this corrosion issue used the same production procedures as before.
We got replacement barrels from Pardini...