Depends on the degree/presence of any pitting.
I wouldn't start with a PB brush, get some Wipeout Patch out and swab the bore then leave to soak, patch out with clean patches then repeat the process until the patches come out clean.
If it is really filthy use Butches Boreshine which effectively laps the barrel.
Once the surface crud has been removed firing a hundred pellets or so will run the barrel in again and fill in any small voids.
If rifle doesn't group after that it is probably fubar.