In thinking this over, trying to come up with a reason, I just don't know. Perhaps I've never seen it, is
I almost always remove the barrel for cleaning. When I don't, I fill the bore with water, vent plugged & let it sit for
10 minutes or so, then shove a patch down the bore hard to blast all the softened fouling out the vent. I do this
several times, then dry and oil. I have never seen that 'type' of 'fouling' on a patch.
Nothing is left in the bore after a shot goes off, except for dry or semi-dry fouling. Nothing with oil, that is for sure. My patches, whether
for target of hunting are WET. Again, I have never seen this.
As Rich said!