Hillbilly,
I have been there and done what you are looking to do. Don't go with the 22 cal heavy bullet, Barrel life sucks, been there. I have had 3 6AI they work, but are hard on barrels too. After the big 6's, I went to the 6BR 12tw shooting 70gr Sierria Blitzkings great 600 to 700yd gun. I now shoot a 6BR, 12 and 8tw using a 87gr Vmax. The 87gr Vmax is the best GH bullet out there, it has 0.400 bc, great long range bullet.
Your 12 tw should shoot the 87 max, mine does. The 80 Berger are a very accurate bullet, but lack punch down range. The first kill I had with the 87gr Vmax, was at 500 yds. the groundhog was half way out of the hole. At the shot it pulled the GH out of the hole with a backwards flip landing 3 ft from the hole. The 87 Vmax hit hard, very hard. The 87max are as accurate as the 80 Berger, they will run 0.5 moa in my guns.
I have not had any luck with the 105gr bullets lots of shoot thru's. maybe in a big 6mm
If you are looking longer range gun try the 87gr Vmax in your BR first. If you want something different, build it around the 87 Vmax.
good luck with the project.
Mark Schronce