I found this process several years ago reading an old book on rifles.
NOTE: J B Compound will not remove the steel but will only polish the bore. You need something more abrasive if you have to remove heavy pits. Of course this will also roung off the rifling edges more too. Lap only as much as necessary.
1. I pushed two patches down the bore wetted with #13 BP solvent (but plain water will do) This is a step I put in because I didn't want any molten lead going all the way down the barrel and possibly getting hung up at the flashhole. Dont worry though, the lead will not stick to the steel.
2. Take a clean or new bronze brush and apply some flux to it and slide it down the bore leaving just thethreads stick out. (To attach the ram rod later.)
3. Melt down the lead getting it good an hot. I just put 3 or 4 .54 cal balls in my pouring ladel and use a hand torch).
4. When the lead is getting a bluish tint to it, pour it into the bore all around the brush. (I put the barrel in a vise just tight enough to hold it and protected by a leather glove or some wooden shims.)
Don't worry if you get spillover - just try not to get any on the brass threads.
5. Attach your ramrod or range rod to the plug when it cools (solidifies) and pull it out. Trim off any excess lead with wire cutters and file a bevel on the back end (threaded end). The cold barrel may not allow the lead to flow all the way to the far end of the brush. But that's fine. You only need about an inch of plug. You can cut off the excess brush if you wish.
6. Berfore you begin lapping, be sure to get a patch worm and pull out the wet patches.
7. Run an oil patch down the bore.
8. Now you can begin lapping. I like to use a mild abrasive like JB compound but you can use auto polishing compound and for really bad barrels bathroom cleanser (Comet). But be careful, this stuff is really abrasive).
9. When you apply the abraisive, CAREFULLY ALIGN the plug with the rifling in the bore.I like to start it with my fingers before I screw on the range rod. When the plug and rifling of the bore alignyou will freel it engage (like gears). Remember, this plug is ONLY for this bore.
10. During the lapping it is best to use a range rod with a T handle that you can loosen and allow the plug to follow the rifling as it moves in and out of the bore.
I usually lap a little run some clean patchs down, inspect, repeat as necessary.
If done properly the lapping will have a minimal effect on the rifling. I'm sure it does remove some of it and it may round the edges of it a bit but IMO its better than a rusty barrel.