When evaluating range claims, there are two things to consider.
The first is the terminal ballistic payload of the round fired. It leaves the muzzle with a certain amount of energy, and starts shedding it immediately. One thousand yards later, will it arrive with enough energy to be useful?
The next is the accuracy of the rifle itself. Let's assume you've got an average, 3 MOA rifle.
100 yards away, you're 3 inches off your point of aim. 200, 6 inches, 300, 12 inches, 400, 15 inches, 500, 18", 600, 21", 700 24", 800, 27", 900, 30", 1000, 33".
Now, how big is your target? I'm an average American male in most dimensions. I'm taller than 33", but I'm not 33" wide. Assuming you can manage windage and holder at that range, is 3 MOA enough to reliably hit a human sized target at that range? How about 2 MOA?
A practical item to take into consideration is the holdover. The holdover for .308 @ 1000 yards is about 34 feet, whereas the holdover for 7,62x39 is about 12 feet at _500_ yards. It's not easy to imagine a point 34 feet above a point 1000 yards away, and so one hopes that your sights have a lot of adjustability to them.