Re: new build....help pick the caliber
My approach would be to get a .22-250, or a .308 and rebarrel to .22-250. This chambering is capable of being loaded according to .223 and .22-250 performance specs and anything in between, without needing to do anything special about brass, etc. For the 75gr and somewhat heavier, the rifling twist neeeds to be at least 1:9" and might do better with 1:8". Really lightweight bullets might be at a disadvantage at peak load velocities, in that the fast twist could result in bullet disintegration due to exessive spin.
Folks will also point out that the .22-250 can be punishing on the bore/throat, but that's only especially true when loading the case to its peak potential, which is decidedly not absolutely necessary. The chambering also lends itself to tailering such loads downward while still maintaining very decent performance.
Such flexibility can be a particularly valuable asset.
I went through a bad patch a few years ago and had to sell a number of cherished rifles. But of all the ones I sold, the only one I honestly miss is the .22-250.
Greg