I'm skeered to drill a barrel except for the touch hole.
While I agree with the gentlemen above and their sage advice, I will note that it is common to drill holes in barrels and thread them and have no safety problems. Examples are drums, touch hole liners, and often patent breeches will have a hole drilled clear through, with the other side threaded for a clean out screw or just a plug. These are at the breech where pressures are highest if I recollect. So I am not as concerned about safety. I would worry about accuracy, forming a spot for rust etc to start and catch the patch and cleaning tow on the way in and the way out. And a gun with a safe and effective repair done at an underlug would have to remain my personal gun.
I won't mind if others disagree with this idea and you should weigh all the advice. IF I was to attempt to repair it and fix the rifling, I would determine if the hole is in a groove or on a lang. If on a land, it's pretty much a goner as far as I' can figure. If in a groove, I would thread the hole and countersink it. In the countersink I would engrave a few furrows radially to the hole. Then pour a slug for freshing or lapping. I'd lap the barrel w/o any screw in it to remove burs. I would gently round the "in" end of the screw, just dome it a touch. Then I'd begin to turn the plug screw in till it touched the slug when it passes. I'd lap it, turning the screw no more then 1/12 of a turn, lap like crazy, pour a new lap, repeat till you've turned it half a turn and you don't have resistance. With the barrel unbreeched and in a bright light, check things out. If/when you think it is OK, cut off the external part of the screw a little long and peen it down into the furrowed countersink. It should never move.
Just brainstorming here, how a barrel COULD be saved if you had to.