As far as i can see, you only have two choices: lap it, or buy a new one and start over.
A friend almost ruined his .40 cal Getz barreled longrifle by using boiling water to clean it. Naturally it flash rusted each time he cleaned it, so the frosting of the bore increased every time he cleaned it, to the point that it was difficult to load, and really difficult to clean properly. I acquired the rifle when he died and discovered the damage. So I set about to lap it.
I turned a lapping plug out of a steel bridge spike: 12" long x 3/8" di. The plug is 5 1/2" long overall, just under 3/8" at the big end and just a little bigger than the 8 x 32 tpi threaded small end. Both ends are threaded 8 x 32 tpi. On the small end goes a washer and a nut. The big end goes into your lapping rod which is best if it has a ball bearing handle so that it can rotate with the rifling unobstructed by your grip on the handle.
I cast a pure lead lap around the plug in the muzzle of the barrel, pushed out the lap enough to trim it so that the threaded end of the steel plug would take the washer and the nut, and then pulled it down to the breech end of the barrel and withdrew it enough to oil it and charge it with valve grinding compound. I ran the lap back and forth in the barrel until it floated without resistance, then repeated the charging and lapping until it would cut no more. Then I cleaned the lap, pushed it to the muzzle, gave the nut a quarter turn, returned it to the breech where I charged it again with valve grinding compound, and continued the lapping process. After about two hours of this, I changed to fine valve grinding compound, and continued lapping.
I finished with some 600 grit emery powder. Over the course of the process, the lap advanced down the plug about 1/8", sliding up the taper and expanding each time to tightly fill the rifling. I cut a groove on one side of the lap's end as a reference mark to correspond to the front sight, so that when I pushed the lap right out for cleaning it and the barrel, to check the progress, I could return it to the barrel the same way it was cast. My brother Daryl was an enormous help with this project, and we spelled each other off when necessary to continue the process over the course of about 6 hours.
The result was entirely worth the effort. This barrel has a gold band around the breech and a gold name plate set into the top flat, and replacing it was hardly an option. The result of the lapping was to completely rejuvenate the bore. Once again, it is easy to load, and is deadly accurate. It still is not as easy to clean as a new barrel, or one that was cared for properly, but so what? The results of lapping are profound!
So two things: one - don't use boiling water to clean your barrel...tepid or even cool water does just as good a job without the risk of flash rusting because of the too rapid evaporation. And two: you can save a rough barrel by through lapping.
In your case, you are likely going to run the lap right to the muzzle for a while to further polish the bottoms of the grooves, and spend almost all of your time working the breech. A mark or stop on the rod will tell you when you have gone in deep enough.
Good luck and let us know how it went.
