Totally clear now. Took us a while to get there. Seems like the only thing to do is wait to see what Barrett will (or will not) do about your gun and the chronic fouling issue.
What if Barrett says, "Gun is fine. Just clean it. Have a nice day."
Will you?
1) Keep it or
2) Sell it
This has to be frustrating. You HAD a great shooting Tikka T3. Now you have a Barrett that is only "good shooting" and then, only for about 6 rounds then it goes south (with ANY bullet...got it now
)
If Barrett asks for the rifle back, then you are getting a 'do-over' and will only know what you have once you get the gun back.
If you are
emotionally attached to this Barrett, and they do nothing, I stand by my recommendation to try the FinalFinish bullets to smooth your bore and reduce the fouling. Then you might consider Boron Nitride (TBN) coating your bullets.
If it were me, and I got no satisfaction from Barrett, I think I'd sell the gun, buy another Tikka T3 and move on with life.