Try soaking the barrel in FP10 for a few days. It is a CLP so you can leave in there as long as you like without damage. It will tunnel under debris and lift it off the surface. This is done by mechanical displacement, not being dissolved by a solvent, so it takes longer. I'm doing this right now with a couple rifles that have not been cleaned for longer than I would care to admit.
Just put 2 heavily soaked patches through the bore, then brush with several strokes. 2 more heavily soaked patches and let sit for a while (at least 24 hours, but longer is better). Come back later and repeat. I have done this a few times now and the patches continue to come out very dark each time. Still haven't got everything out.
This method takes time, but little effort since the FP10 does most of the work.