I'd start by plugging it and filling the bore with evaporust to remove the rust chemically without affecting the steel.  Then, brush with bronze to clean up the carbon on the surface left by the rust removal.  Then, push a tight patch through and see if you can determine where tight/rough spots are.

At that point, knowing the tight spots, I'd lap with JB bore paste on a tight patch only in the observed tight spots, and once they seem better, do the entire barrel more.   You always want to overlap the final lapping sequence from the breech end, to taper lap the bore towards the muzzle.  Start with lapping from the breech forward 1/3 of the barrel length.  After 30 to 50 strokes, reloading with JB when it doesn't seem to be "cutting", do another sequence at 2/3 length, and finish with a sequence going right to the muzzle.