What do your groups look like with other guns that weigh 12 lbs or less? If they are better, the list is pretty endless but includes bedding, absurd bedding issues like trigger contact with stock inlet, action screws too long, scope base screws too long (not sure if this is applicable to this particular action or not), scope failure, mount failure, bad batch of bullets and on and on. If you are shooting other 20lb comp guns into tiny groups then you are fooling yourself. A 20 lb pop gun will almost shoot itself while a 12 lb gun is pretty critical in comparison.
If you want to test accuracy of the system for not $1000, load some Berger 140 bt target over 4350 in lapua brass. If that doesn't shoot, it is something with the gun.
Before shooting more than 5 shots was main stream, I used to say the best way to turn a 1/2 moa gun into a quarter moa gun (5 shot groups) is to switch to Berger bullets. I do not hunt with Berger often because I don't love the terminal performance, but there is almost always an improvement in precision. The 140 bt target is a very forgiving bullet in modern chamber designs. Any mild charge that doesn't crater primers and any reasonable seating depth should shoot with this combo. A good blank, good smith work, good brass and bullets, and things normally shoot lights out for prs/hunting/nrl no matter what the specifics of the load are. If there are no equipment issues, you are probably looking at a crooked or under/over sized chamber. CNC has soured a few really good gunsmith shops. It doesn't matter how consistent the computer runs the tooling if the bore and the tooling are not perfectly concentric.
I have turned a custom 243AI hunting barrel from 1 moa (5 shots) 1.5 (10 shots) to .5 moa (5 shots) 1 moa (as many shots as you want to shoot) with Hornady 105 hpbt just by sorting the bullets by OAL. The bullet quality makes a lot bigger difference than most people think, especially if it is bad.
The other thing I would consider is to dump it and start over, or send it to a GOOD gunsmith to sort out. It will likely be cheaper than putting a barrel's worth of loads through it only to spend your time,$, and frustration. After having my teeth kicked in just like this several times, I stopped worrying about the money. I contact the builder and work with them to solve the problem. If they are helpful (most try) then we work together to solve it. If they are not receptive, I cut and run and start with someone else that I pay to sort it out. You can run almost an entire rifle worth of $$$$ through a barrel trying to figure it out or you can spend that money on something that will shoot well and enjoy it.
My $0.02, hopefully something in there is helpful.
