I know a gunsmith near me that uses proof or carbon six on all his rifles, and constantly posts pics of the accuracy he is getting with them, and they seem to shoot excellent. I would say if he is having accuracy issues on his second proof barrel, he needs to look other places than the barrel.
Base screws contacting barrel tennon
Base/scope ring screws not properly torqued
Action screws contacting bolt head
Action screws not properly torqued
Scope issues
Action bedding contacting trigger housing or trigger pins
Barrel pressure
If it has a muzzle device, was it installed properly (try shooting without it on)
How competent is the gunsmith that assembled the rifle
Is good loading technique being used
Is proper shooting form being used
Have you tried shooting off bags vs bipod if it has only been shot off bipod
Just a few things to think about. Not saying any of these are the cause, just a checklist I myself go through when trouble shooting rifle accuracy. And you may have very well gone through and double checked all of these, but if I went through two premium (and proven) quality barrels and accuracy is still poor, while not impossible, I would HIGHLY doubt that a barrel is at fault.