Re: AR10 build
The 308/7.62x51 is hard to beat in the AR-10 whether you reload or not. It is a tried and true performer for long range target out to 800yds (and further depending on your ammo and skill set).
If you do reload, there are a host of different offerings available these days that offer a ballistic edge over the 308, especially the WSM cartridges including 6.5WSM, 7WSM, 300WSM, etc. The 6.5CM is also a great round that will work in the AR-10 platform. However, none of those rounds really have much in the way or commercial ammo offerings that will allow you to get the most out of the cartridge in the AR-10, so you have to reload if you go down that road.