Quoted:
Why don't you try one of the various bore pastes first? J-B Bore Cleaning Compound or USP Bore Paste are two brands I know of. They are mildly abrasive and work very well.
View Quote
I second the suggestion for JB. It might take more time than fire-lapping, but should do OK if you're not in a rush.
I would suggest that you give it a good overnight soak in Hoppe's, then push out as much as you can. Dry the bore. Then give it a good scrub (back and forth, moving an inch or so farther down the bore each stroke) with JB. Push out as much as you can to clear the bore.
At this point, either soak it with Kroil if you've got it, or more Hoppe's. Continue alternating the same way, with Hoppe's in the bore for overnights. When the bore feels smooth to a gentle touch on the cleaning rod, the time for JB is over, and it's time to just use Hoppe's or a copper cutter for a while.
If you suspect corrosive salts in the bore, alternate in some Windex or Simple Green, or mop it out with hot water. Any of those should be immediately dried out though, followed by reapplication of a protective soak of Hoppe's.
I had really good luck doing this with a Turkish Mauser. Looked like a sewer pipe inside, no visible rifling at all, after I pushed out the tar/cosmoline. I spent three weeks cleaning it, each day pushing out the stuff loosened by the Hoppe's, then soaking with Kroil, doing something else for a while, then a quick patch to get the excess Kroil, then the JB scrub, then push out the remainders, then resoak with Hoppe's for the night. Came out nice and shiny, with sharp rifling and a smooth mirrory bore.
Of course, 80 rounds of Turk 8mm ammo, with the silvery looking gilding metal on the bullets, and it was all nasty again, requiring another two days' cleaning on this regimen. From now on, it gets only Greek surplus ammo.