I've had barrels that were almost completely shot out do way better than that. What I mean by shot out is the lands were severely worn, rounded and almost nonexistant.
Best guess would be some sort of severe crown damage that is damaging the bullet on the way out. I have a rifle now with a noticeable crown ding, it still shoots straight at a hundred yards
A borescope may reveal something. Though if it is shooting that bad the damage would likely be visible with a flashlight and eyeball.
Maybe try different ammo,