My deer rifle for years was the 6.5x47 Lapua. So my choice for my first AR-15 was 6.5 Grendel. I shoot the same bullets with some reduction in velocity, which is still more than adequate for deer & hog.
I am just finishing up my 2nd AR-15. This one is a wildcat, the 358Yeti. It is based on the .308 Win case shortened and expanded to 358.
It will launch 180gr bullets at 2,600 fps and 200gr at 2,400fps.
