That barrels wrecked, and will throw fliers BUT, I have a few rifles, 1 with the same exact bore you have, and 2 more that have REAL bad bores, and surprisingly they all shoot pretty dam good with handloads lol. At a hundred, I can keep em inside of two inches, and the one that looks like yours, is a one moa gun believe it or not (yugo m48)
What I did to mine was clean the SHIT out of them, which actually takes a few hours, then I shot the Tubb lapping bullets through em, and surprisingly now they shoot pretty good. Granted those bullets only actually cleaned up the first third of the bore, but they did actually help my groups. Unfortunately for your milsurp, that's the exact caliber that if on the higher tolerance of bore diameter, does not have a decent selection of bullets to try out (higher end of .312", which some are .313" ID) for a good bore/bullet seal. I'd still try er out though.
Try some Norma brass with the .312 Hornady 150 gr flat bases or any of the heavier .311 copper solids at inside of 300 yards.
These wrecked barrels copper and foul BAD, and always need 3 times the amount of cleaning as all my other barrels do, but I'm currently in an experiment where I just go after the carbon and leave the copper, to see if that helps with gas seal.
For your initial clean, wet it in butch's bore shine and let it sit for a couple of minutes, then attack it with a steel bristled brush a few strokes, then wipe all that out and continue with the normal bronze bristled brushes.