Cowboybart,
I use my 1-14 twist 6mm PPC benchrest rifle for prairie dog shooting occasionally, using 55-grain Ballistic Tips at 3600 fps or 65-grain hollow-points at 3400 (it has a 25-1/2" barrel). Haven't tried to shoot any at 600 though it works fine at 500. But if building one specifically for your purposes would do a 6mm BR instead, for the convenience of the .473" case head and a little more zip.
