Ditto to Boz's suggestion.
More often than not it's going to be a shooter induced error of some type when your having your groups open up that much.
Is your rig free floated and bedded?
Are you shooting from the same position when going long vs the 100yds groups?
What scope are you shooting? Is it parallax adjustable and are you adjusting it properly for the long shots?
A sub MOA rifle and load should remain sub MOA between 100-600yds without any problem.
Last question, how much time is transpiring between shots at 100 and 600yds and are you letting it cool down completely after finishing at 100 before starting on the 600yds target.