There are two 600 yard benchrest matches scheduled at Benchrest Rifle Club this year - 22 Feb and 21 Nov

IBS rules (except factory class guns)
4 targets of 5 shots for score

Contact Bill Meuser, PO Box 373, Dittmer, Mo 63023 for info.

Now this seems interesting to me - with the dominance of wind at this range, it seems to me an accurate rifle with a conventional stock and shooting an appropriate cartridge ought to have a good shot at competing against the pure benchrest rifles.