No wife's AR is 5.56; obviously lighter ammunition requiring less bulky parts which means less weight.
If you want to spend the money, I just saw a .308 rifle on youtube that looked pretty sick…$5,000
H&K MR762A1LRP
https://www.youtube.com/watch?v=H82xJvh9SE0
If you do a lot of digging around and actually researching parts to build with you can probably build a nice one. Also check out fultonarmory.com they have some nice guns there