The scope is not that critical. What is critical at long range is knowing the (nearly) exact distance.
Suppose an elk at 250 yards that you misjudge by 50 yards (you think it is 200 yds, or 300 yds. Assuming an accurate round shot off a good rest, you will probably still kill that animal, with either mistake.
Now assume the elk is 650 yds away and you make the same 50 yd error (you think it is 600, or 700). You will probably completely miss the animal.
Please also do not make the mistake of thinking that because you can shoot 6" groups at 600 yds off a bench means you are good to go on elk at 600 yds. The difference between the best field rest and a benchrest is usually significant.
Suppose an elk at 250 yards that you misjudge by 50 yards (you think it is 200 yds, or 300 yds. Assuming an accurate round shot off a good rest, you will probably still kill that animal, with either mistake.
Now assume the elk is 650 yds away and you make the same 50 yd error (you think it is 600, or 700). You will probably completely miss the animal.
Please also do not make the mistake of thinking that because you can shoot 6" groups at 600 yds off a bench means you are good to go on elk at 600 yds. The difference between the best field rest and a benchrest is usually significant.