Re: New rifle build...which actions
I've only been to two 600yd matches so far and one was won with a Surgeon 591 and the other a trued Remington 700. The Surgeon posted a score of 286 out of 300 with significant wind. The Remington scored a 290 with mild wind. Other actions scoring in the mid-270s to low-280s ranged from savages and other Remington clones.
On the flip side, I have seen other Surgeons, Remingtons, DTAs (me), suck ass. A good smith can put together a rifle that will shoot regardless of action. As long as you get a good smith and are a capable shooter, I wouldn't worry about the potential accuracy differences between each action.