Without an inclined scope base like this one, the Leupold 3.5-10x40mm M1 is limited to 850-900 yards shooting standard M118LR 7.62x51 ammunition.
How much magnification do you need? The answer is less than you might think. The scope needs to provide a clear image of the target for a well-defined sight picture. With good glass, a 12x scope is more than sufficient for 1000-yard shooting. To test the theory, I dialed down my 3-12x50 mm Schmidt & Bender scope to 4x and was able to make hits on a 36-inch target at 1300 yards; it was enough to discern the target from the background. A lot of people think they need very high magnification to shoot at long range, however, as the magnification is increased, the field of view gets smaller and it is harder to find the target in the scope. Another problem at high power is that the size of the exit pupil shrinks, which makes eye position a lot more critical for a full view through the scope. A smaller exit pupil also makes it harder to track the target though recoil, which is how a shooter can spot his own bullet impacts. At the low end, dialing down the magnification makes it easier to find close and track moving targets, and provides a brighter image in low light. So what's the answer?
For practical shooting to about 1000 yards, a top end of 12x to 16x gets the job done and puts the low end between 3x and 5x for most scopes. Another reason to pick the lowest magnification that will work for the longest shots is to help limit rifle weight and bulk.
The Leupold Mark 4 line is a good value that gets the job done, but lacks some features of the higher-end scopes. Shown here is a 4.5-14x50mm M1.
There are four scope brands that are particularly common at long-range training and competitions: Leupold, Nightforce, US Optics, and Schmidt & Bender. There are cheaper choices, but I recommend the Leupold Mark 4 line as the baseline choice because it provides the basic features required, is proven by years of duty use, is unlikely to break, and is backed by an excellent warranty.
Leupold's Mark 4 line of long-range scopes includes four variable-magnification models: 3.5-10x40 mm, 4.5-14x50 mm, 6.5-20x50 mm, and 8.5-25x50 mm. They have between 65 and 75 MOA total elevation adjustment which is enough to get .308 to 1000 yards if an inclined base is used. All are available with the M1 knob, which has fourth-MOA clicks in 15-MOA per turn and has no zero stop. The 3.5-10x40 mm models are also available with the M2 and M3 elevation knob, which has one-half or one-MOA clicks. The single-turn M3 knob also has a zero-stop. Leupold only offers mil-based clicks in the 4.5-14x50mm "metric", which has six mils per turn of the M1 knob in 0.1 mil clicks. This scope is the only option for matching click units to reticle units since they have no MOA-based reticles. First focal-plane (FFP) reticles are only available on the 3.5-10x40 mm and the 8.5-25x50 mm.