It probably is important to recognize that with some scrubbing, significantly stained (I presume baked on powder/oil/lead residue?) barrels can be impressively cleaned up.
This is the as received Remington A grade evaluated some time ago. The gun was dropped in the snow after the burst and not cleaned.

[Linked Image from photos.smugmug.com]

After wrapping a Big 45 Frontier Cleaner pad on a rod, chucking it in a cordless drill, soaking in Kleen Bore's Formula 3 Gun Conditioner and goin' to town

[Linked Image from photos.smugmug.com]

[Linked Image from photos.smugmug.com]

Certainly not a "mirror" bore, and tiny pits are evident.

And without aggressive cleaning, one can not know that might be under the staining. The Southern 16g blow-up gun.

[Linked Image from photos.smugmug.com]