Has it got a mid-barrel bulge? You'd feel the different resistance to a brush as you push/pull it through. The dark section of the bore is usually caused by pitting, not bulging.
Before doing anything extreme to the barrel, don't write the bore off until you shoot good ammunition through it, not some old milsurp. Rifles usually age well but their ammunition less so. Also, the dark middle of the bore may have been present even when the rifle was a tack driver, it does not happen overnight even if the rifle lies unloved for 20 years.
I'd happily sell you a used replacement barrel, but ... caveat emptor ... will it shoot any better? Don't buy a car or a milsurp barrel from a friend. To get a dark old milsurp bore shooting better, first look into lead lapping.
Article: "Lapping a Rifle Barrel", Shooting Times, September 23, 2010.
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