plug up the muzzle, fill the bore full of Kroil, and let it sit overnight. Next day, dump the kroil, brush the bore good, swab it clean, and load it up and shoot it. I have found the larger bores more tolerant to a less than perfect bore than the small calibres.
For the average sized animals that are usually hunted with a 338, anything under 2" is "hunting accuracy".
I'm curious, did you go back to the seller and ask him about any of this?