BTW, you can lap your own barrel blank before you chamber and crown it... so if marks in a barrel really bother you, you can do something about it. Start at about 800 grit lapping compound and work your way to finer grits. Takes at most a couple hours.
True, but one needs to do something else first. . . .
Buy an air gauge with its gauging head matched to your bore and groove profile, or, buy a hole micrometer accurate to sub 1/10,000th inch long enough to reach the middle of the bore from either end.
Learn how to use either bore gauge so you get repeatable numbers measuring the same point in the bore.
Learn how to make a lapping head out of some soft metal that matches your bore and groove profile. Then make one.
Learn how to charge that lap with the right compound and use it to best advantage. Got a barrel to use as a training aid to practice with?
Get the feel of lapping the bore and measuring its diameters from breech to muzzle so they're at least as uniform as the original bore. Use that barrel you practiced lapping with.
And finally accept the fact the the lapped bore will have greater bore and groove diameters than it originally did.
Otherwise. . . . you'll probably make that barrel not as good as it was in the beginning. But it will have the smoothest finish on all those hill and valley high and low spots that hurt accuracy.