Turret adjustments should be distinct with tactile and audible clicks. Most important is the need for the scope to track consistently throughout its adjustment range.

When choosing the magnification range of a variable scope you must also take into account the ambient lighting conditions. Why? Simply because magnification and objective lens size need to be considered together as they interrelate. If you never plan on shooting in any conditions except bright light, this is not an issue for you. For the rest of us exit pupil size is very important. Ideally for low light use you want your scope to generate at least a 6mm exit pupil that will allow you to locate, identify, and engage your target. Exit pupil is calculated by simply dividing the objective lens diameter by the magnification. As an example, a scope with a 40mm objective lens set at 10X generates a 4mm exit pupil. Power that same scope down to 5X and it generates an 8mm exit pupil.

Also, theoretically, the larger the objective lens the better the resolution (all things being equal). Enhanced resolution is a useful aid when shooting under low light conditions.

Objective Lens Size
While I have shot very well under moonlight using scopes with 56mm objective lenses, please keep in mind that large scopes have drawbacks. They're heavier, bulkier, and easier to knock about. Plus, the larger the objective lens the higher the scope will need to be mounted above the bore. From a tactical standpoint, the larger the lens the more likely it is to reflect light and disclose your position.

Tube Diameter
Tactical scopes are commonly available with 1-inch, 30mm, 34mm, and 35mm tube diameters. Generally, the larger the tube diameter the more robust the tube is and the more adjustment the scope will have. Larger tubes with thicker walls also add weight, so there is no free lunch. By far 30mm is the most common diameter on tactical scopes, with Schmidt & Bender offering 34mm and I.O.R. offering 35mm models.

Adjustment Turrets
Mounted to the tube will be the scope's mechanism block with the windage and elevation turrets. These control the ballistic adjustments you dial into your scope. The turrets should provide very distinct audible and tactile clicks so you can easily keep track as you make adjustments. Scopes are commonly available with 1/4-, 1/2-, and 1-MOA adjustments and some offer models with 1/2 and 1cm adjustments. I find 1/4-MOA adjustments too fine for field use and 1-MOA adjustments on the coarse side. I prefer 1/2-MOA adjustments because they cut the number of clicks one has to make (and keep track of) in half. If you are willing to range in meters and do all your ballistic and Mil calculations in metric, a scope with 1cm adjustments will also work very well. I prefer capped turrets, a positive zero stop with all elevation in one full turret rotation. The adjustment range of the scope should be more than enough to reach your maximum desired range with your load of choice.

When choosing a tactical scope, David recommends picking a rugged design with reliable adjustments, superior light transmission, and excellent resolution.

Reticle
It seems as though everyone has a proprietary tactical reticle today, so wading through all of them to find one that's right for you can be difficult. Instead, focus on your actual needs and then pick a suitable design. To start, a reticle needs to have a definite easy-to-define aiming point that is plainly visible in low light or shadows. It should also incorporate at least one method for quick and accurate rangefinding. Holdover and lead marks can also be useful, but the last thing you want is a cluttered FOV. Whatever you decide on for a reticle, I strongly suggest you opt for an illuminated version. Properly utilized it will greatly aid low-light shooting.

Should the reticle be in the first or second focal plane? Again, this is your choice. There are pros and cons to both designs. A first focal plane reticle can Mil or rangefind at any magnification. A second focal plane reticle is easier to employ on low magnification and in low light.

Parallax Adjustment
When it comes to parallax adjustments, side focus knobs are currently the rage. Despite this an adjustable objective (AO) has the advantage of being both more precise and more robust. In many ways it's the better design, but AOs are no longer hip and are rapidly fading from the scene. I do recommend a European-style fast focus eyepiece. This is a nice feature to have.

While the perfect tactical scope does not exist, there are some very good designs available. I recommend carefully evaluating what your needs are then picking a professional-grade optical instrument from one of the top-tier optical manufacturers. Once you acquire it don't forget to test it to ensure the tracking is true. Then practice with it so you know it intimately.