Re: new custom cant get to shoot
What's done is done.
The good news is still the fact that you have a single shot load that shoots accurately.
Do the mag well modification that Jim See recommended. If you aren't sure how to get it done, I'm certain he can do it for you.
You're basically out the extra shipping cost. But, you would've had to pay for materials and installation anyway. So, while you could've gone with a long action or a short throat, it sounds like you'll end up with the best option using the new mag well regardless of how you got there.
And, don't overlook the scope base alignment issue as pointed out by some.
Simply changing bullets and/or playing with seating depths may get you where you need to be. But, at the expense of barrel life and cost of components. So, the mag well is probably a safer bet at this point if you truly have a load that shoots one ragged hole and if it wasn't just a one group fluke.
The load workup need not be complete guesswork either. Apparently the Berger will shoot fine in this rifle when seated near the lands.
But, I would also strongly consider the primer/powder recommendations that others have suggested. ..particularly as pertains to H4350 or H4831sc as they are considered to be consistent performers.
Then, start with a methodical load workup and do not cut corners.
Let us know how it turns out.