Once a rifle is grouping larger than 3" it's hard to tell.
Have seen completely shot out barrels with virtually no rifling left still shooting 3"
If there is a sighting problem, that could do it. But sights should be on barrel and upper.
I saw one case (savage rifle) where the wrong recoil lug was used which had a pin and the pin bottomed out before the nut tightened on the action and the previous owner had been shooting it like that. I got it as a total basket case, even though it had a select match barrel and custom trigger.
I had to remove the barrel, get the correct recoil lug put it back together and re-headspace it, then fasten the nut and re-check. That rifle shoots groups with holes overlapping, near 1 shot groups
So I would agree that very likely something is terribly wrong with the barrel action interface or with the barrel itself. I would get the barrel borescoped and have it recorded. Could be chamber is way eccentric to bore or a multitude of sins. If you saw some of the chamber casts of TC chambers and others it is no surprise some barrels won't shoot.