Re: which one?
I am in the same boat that your are in, looking to get into long range shooting with a semi auto.
IF I do go with a semi rifle, the 260 Remington in the AR10/AR308 platform. Check out the ballistics on the 260 vs 308.
I also was looking at 6.5 Grendel on the AR15 platform, it has better long distance ballistics than the 308.
Right now I am looking into the 338 Lapua in the AR30 and its the same price as a AR10 but the 338 Lapua is a real long range shooter. Do a lot of research before you buy as there are a lot of options out there for a person to get into long range shooting.