Dale,
There's not many 600 yd. ranges around here. Reade range (near Johnstown), Whitehorse (Clarksburg WV), Warrior Mtn. (Cumberland Md.)and Malvern (Ohio) have 600 yd ranges, but they are usually only available for high power matches. McDonald Sportsmen Club has a 600 yd. range, but I don't know the particulars there. There was a pay as you go public range out near Imperial PA that had some long distance, but I believe that it has been closed due to some kind of problem there. Sportsmens Assoc. of Greensburg has a 500 yd. range that I think has target frames for use off the benches. You'd probably have to be a member or a guest there.
Jim