Your eyes are the most important factor in shooting good @ 100 then @600.
Being a Diabetic my vision changes thruout the day and session.
Some days my vision is 20-20 some days are worse. With age/experience you might be a better reloader but your eyes are no longer capable to compete.
I have shot loads from my 308 using a 147 FMJ Win bullet. It stabilizes around 300 yards(2") and the 100 yard groups suck (1-2") sometimes the groups are smaller @ 300!
"Happiness is knowing the Barred Owl is Eating the Spotted Owl and environmentalists are watching Nature take it's course"