Wayne:
For 200-300 yards:
Sierra 75 grain hollow point, flat base
37.5 grains 4895
Reformed Lake City Match cases
CCI 200 primers
For 600 yards:
Sierra 85 grain HPBT or Berger 90 grain FMJ BT (not VLD)
38.5 grains IMR 4320
Reformed Lake City Match Brass
CCI 200 primers
These loads did very well (~ MOA 10-shot prone groups) right out of the starting gates in this rifle, an early Ruger 77V that I modified for XC shooting almost 30 years ago:
This rifle has the factory barrel with 10" twist. Some 243's had a 12" twist; in that case the 90 grain load may not shoot as well. If you have an aftermarket barrel with an 8" twist, there are some very good heavier match bullets that will reach out and touch things past 600 yards.
Resp'y,
Bob S.