You did not specify if the locking bolt was the old solid or the newer spit bolt. If the barrel is used and has the solid bolt I would think it could have been honed to fit another action and thus cause vertical play on your action(s).
My first attempt, if it is a solid bolt, would be to call TC and ask for the newer split bolt. At one time they would furnish split bolts at no charge (and no barrel return was necessary) but this may have changed with all the other changes that have taken place.
Also you can take the bolts from an existing barrel and try them on the 256. I have changed the bolts on barrels where one worked and one did not and after the swap they both worked fine.
Ken