Not to state the obvious, but a round rises, then falls. There are many variables, but on average, it's safe to say a .243 peaks at 100 yards. Then starts falling.. around 200 yards it's around 2.5 inches low, and at 300 yards, it's around 10 inches low. Of you zero at 100 yards, your range of arm accurate group will fall too far below your zero to be consistent. If you zero closer.. say 25 yards, the round will go higher and further along your zero line giving you a calculated elevation difference up closer to 300 yards. Who wants to aim 10 inches high at 300 yards. Unless you want to run towards or away from the target to make sure it's 100 yards away... zero in ther prone at 25 yards.. at least a 3 shot group... adjust until zero. Then set up a target every 50 yards and shoot a 5 shot group to tell the trajectory. And write down the inches in difference. Using a range finder, get used to how far targets are by judging with your eyes. This is for hunting.. anything.
Every weapon varies slightly.. know yours.
USMC Rifle Master and Marksman Coach.