I recently resurrected a WW-II era barrel (mfr date 10-42) that had some rough spots like you describe. It took a while but I got it cleaned up and it shoots great.
Start by plugging the muzzle with a cork and securing the barrel so it's vertical. Fill the bore with a penetrating oil like KROIL or whatever you have in the garage for rusted hardware. Let it soak for a couple days.
Drain the penetrating oil and start with a brand new silicon bronze bore brush and brush until your arm is sore or the brush is worn to the point it just slides through the bore easily. You can also start with a (one caliber smaller) brush wrapped with fine steel wool. The point of this operation is to get all remaining rust and fouling out of the bore.
Now go find a good steel cleaning rod. GI cleaning rods are great for this next step. Also get a couple of extra patch loops that fit the rod. Plug the bore about 1-1/2" down from the muzzle with some moist paper towel. Make it a tight wad. Melt some bullet lead, place one of the patch loops in the bore, holding with a pair of pliers, and pour the lead into the space around the patch loop stopping before it fills the crown of the barrel (if any). Let cool.
Now secure the barrel in a vise using blocks of wood. I use a hole saw that makes a hole slightly smaller than the barrel and use it to drill between two pieces of hardwood with the pilot drill following the part line. Makes a great clamp and doesn't mark up the barrel.
Using a mallet, tap the end of the rod and work the lead plug loose in the bore. From this point on let's call it a "lap".
Working it out of the barrel, apply some valve grinding compound and work it through the bore. Make sure the rod can swivel so the lap follows the rifling. Work with the coarser lapping compound until the bore is smooth then change to the finest. You may have to make another "lap" or two. When finishing you could go all the way to jewelers rouge and have a nice shiny bore if you want to.
Chances are a heavily pitted bore will only become smooth if you do this with some pits remaining. Shooting will foul them first and subsequent rounds will pack the fouling in them. Patches will come out dirty but at least the rifle will be "shootable".
OR just buy a box of "Fire Lapping" bullets from Midway or Brownells. You'll need .311" bullets and I don't know if they're available. If not you'll have to settle for a "Hand Job"