I hit a milk jug at 1 mile this past September. I was using a 4-20 power Sightron with a 50 mm objective on my 6.5 Creedmoor. Some say 10-15 power is all you need, and I do agree, to an extent, but more power is, in my opinion a better way to go. I have seen some of the scopes that range from 4 or 5 power to 25, or 35 power, in my opinion those would be the best choice. You can turn them down, and still have a decent field of you if you plan to hunt with the rifle, or turn them up, and use them for plinking. That's how I run my scopes.
When I hit the milk jug at 1 mile, even at 20 power, it was hard to see that I made the impact. I actually put red food coloring in the water, so that I would see the jug change in color. At that kind of range with a 6.5 caliber bullet, you are probably not going to hear the impact on steel, and with mirage you will probably have a hard time seeing a reaction from the target. That's why colored water worked for me, I was able to see by target change color, knowing that I had made the hit.