I would look at the charts for the ammo you are shooting, but my guess is that the .243 does not carry enough down range power to shoot a whitetail deer 300 yards anyways.
The bullet is too light and the distance is too far.
You want to keep at least 1500 ft lbs of force at the point of impact and a 30-30 Winchester barely has that at the barrel.
Bullet drift in a stiff wind might be as much as 20 inches.