.308 jag + tight fitting cleaning patch + tooth paste/metal polish = lapping tool.
I've never used this on a through-bored gun, but I've done it on many, many traditional muzzleloaders.
Find a patch that creates a nice, tight fit in your bore, once mounted on the correct jag; load it up with toothpaste/polish & run it through your barrel, back & forth 100 times.
Test shoot & repeat as necessary.
It can take a while, but it's much gentler & precise than fire lapping or using scotchbrite.
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