You want a round to have 800 ft/lbs when it hits it's target, I don't know what a 243 carries at 800 yards, but I wouldn't try an 800 yard shot with anything under a 300 magnum, and yes I know other calibers can do it.
As for me personally, my farthest kill was 398 yards with a 30-06 in perfect conditions. I only have like 2 spots that you could even shoot past 250 yards. I'm confident to 400 yards but don't look for those kinds of shots, and I took the average of my shots over the year and it was 66 yards.
