It has been a while ago,20+ years,but I helped some friends develop handloads for their Ruger 77's.At least one was a 30-06.I was very impressed by the accuracy .They all shot groups the size of pocket change at 100 yds.
And,you know,with a rifle that keeps them all at 1 inch,the worst shots are only 1/2 inch from center,which would be about 2 1/2 at 500.That is closer than I can hold,unless I'm on a sandbag on a good day.
I have a book,its loaned out to a young woman,called "Position Rifle Shooting"
To teach something about mirage,the author suggests taking a riflescope to the range early in the morning,while all is still cool.Put a target out at 600 yds,and sandbag that scope on a bench with the crosshairs centered on the target.Then leave it alone,go shoot a few hours.When the sun has climbed up,and warmed the air above the ground,look again.You might be surprised how much the crosshairs "moved"(feet).
That kraigwy guy has forgot more about shooting than I'll ever know,I'd have to agree shooting $1000 worth of ammunition will probably make more difference on whacking those jugs than a different $1000 rifle.
Sure is fun!
You know,also,it could possibly be that a scope just fine for 300 yd heartshots on a deer might contribute a little to groupsize at 600.
It may help to pay close attention to spotweld.Have your eyeball look at the scope the same each shot.You may notice the crosshairs move a little on a target if you move your head around.That is parallax.You can spend money to fix it with a real nice scope,or use a good spotweld will help