sungazer, oversized brush and Hoppes carbon solvent works very well on old carbon deposits.
...but when we're talking old neglected barrels, 50+ years, 100+ years, I find that you need something a lot more agressive.
I found this,
http://big45.com, basically cut a piece off, wrap it around a brush and go to town. Coupled with constant flushing, like boiling water (to start with) and Lannox liberally sprayed onto the bore (bore on an angle so it can drain). This seems to deliver the best results (at least in my case, with very old, very neglected military barrels).