RE: How filthy?
Get some JB Bore Paste. Then with water and soap clean the barrel with patches as best as possible. Then run dry patches through it. When you have it dry, run a patch with some quality gun oil on it. Now smear some of that JB Bore paste on a tight fitting patch and start scrubbing the barrel with that stuff. If anything is going to get down into the pores of that barrel and clean it, that stuff will.
After you have scrubbed that barrel with Bore Paste for a while, give the barrel a good water bath again. Then take a brass bore brush and dip that in solvent and scrub the barrel with a solvent brush for a while. After that run some solvent patches through the barrel.. Finally with dry patches wipe out all the stuff in the barrel as best you can. The barrel should shine at this point. You have taken it back to the metal base. So be sure to end the session with a patch and some quality gun oil and really swab the barrel good, working that oil into the pores of the barrel. You should be able to examine the bore of the rifle now for some damage.
If there is slight pitting, it might not be the end of the world. Take the rifle out, load 100 grains of Pyrodex RS loose powder and a 250 grain shockwave and see how it shoots for you at say... 50 yards. Go from there and work up some loads.