Well I have brought back several guns from wallhanger status to usable, but that sounds pretty bad. Land diameter enlargement towards the muzzle means gas blow-by, but the bullet is still being held on course by the grooves, so the rifle may still shoot straight. With black powder rifles one can pack in a lot of lube, so that the hydraulic seal is (hopefully) maintained right up to the muzzle. However, that is not a solution that I have heard of being applied to smokeless rifles.
Groove diameter enlargement as well means the bullet is now longer axially guided as it approaches the muzzle. So it will develop a yaw before it emerges, and that will probably cause tumbling. I would not spend too much time on it.
:wave:
Patrick