338Freak,
It sounds like you have proved the problem isn't with the can. IF the groups are the same with, and without the can, it isn't the problem.
Your 6.5 shoots fine, so there isn't a problem with the chassis or trigger. That leaves the 338 bolt or barrel. With an accuracy issue, the barrel is the suspect.
You have tried different bullets. Which powders have you tried with the 338? If you tried Retumbo or H1000, I think the barrel is the issue.
It sounds like you have proved the problem isn't with the can. IF the groups are the same with, and without the can, it isn't the problem.
Your 6.5 shoots fine, so there isn't a problem with the chassis or trigger. That leaves the 338 bolt or barrel. With an accuracy issue, the barrel is the suspect.
You have tried different bullets. Which powders have you tried with the 338? If you tried Retumbo or H1000, I think the barrel is the issue.