What makes a rifle capable taking PD's at distance is the BC of the bullet. It takes very little energy to kill a PD. Here is a great story on a 20BR. Tennessee Triple — VolDoc’s Varminters within AccurateShooter.com
I would look hard at something shooting the 20.cal 55gr berger varmints.
If your sold on a 6mm look at the benchrest rounds, Like the 6mmBR, 6XC, or 6mm Dasher, along with the high BC 100gr + bullets. Much better accuracy, less recoil and better barrel life.