Not being critical by any means, but I don't know about swapping out bolts from one rifle to another on a custom gun, or any other gun for that matter, for me each gun is an entity within itself. With that said, the bolt from your other rifle may not be within SAAMI specs for your rifle. The last time that I had a similar problem I found that it was the firing pin spring. Instead of using the 17 pound spring, I replaced the spring with a 24lb Wolffe spring, that solved my problem. That's when I read on the forum that the older Ruger 77s tang safeties had a firing pin spring problem when they got older. I threw this out, however I don't think that this is what is going on with your rifle.
I'd also suggest to you that I would be sending the rifle back to the gunsmith. I don't know what your rifle cost you; however I suspect that it wasn't a "cheap date", and the work was not done for free either! The gunsmith built the rifle and it "is" his/their problem not yours, he ought to be working with you to make it shoot as it should. One question that I have for you, how did the rifle shoot when it was a 26 Nosler and before the work was performed on it???