If you look at the NRA's rule changes effective 2017, they do state that unlimited sighters are now permissible for 600 yd prone slow fire. And I do know of a record string that was shot at 600 yds with unlimited sighters that the NRA accepted and the shooter was awarded national record recognition.
Well then, there you go. If the NRA changed the rule and just hasn’t put it into the rule book yet, then I guess I have to retract all of my previous comments.
It does make me wonder how we managed to make it all the way to 2017 only being able to shoot 2 sighters for mid range though.