Actually, the scope specs should read: 6.5-20x50 not 6.5x20x50. 6.5-20 means that the magnification is adjustable from 6.5 times your eyesight to 20 times your eyesight (or how things appear to you). Thus, you're not multiplying 6.5 and 20 as many ignorant folks will say/write it. The spec is actually saying the scope will adjust from 6.5 power to 20 power. The 50 has to do with the size of lens toward the muzzle (objective). It stands for 50mm, which is the width of the lens. The larger the lens, the more light will come in. However, the larger the lens, the taller the scope mounts, and therefore a taller cheekrest is often needed. Most folks don't need a 50mm objective lens. My personal opinion is that for a tactical scope, you want either a mil-dot or a rangefinding scope. The duplex does not offer any type of rangefinding potential, short of the Tasco 30/30 style of rangefinding. Mil-dot types will tell you how far away something is as long as you know the item's height and the amout of area that your dots equal at a certain distance. A little math and you know how far it is away...then you make the appropriate click adjustments to match your bullet drop at that distance. What I just said is very simplified compared to what's involved, but if a Marine can do it, anyone can...right?
On the rangefinding types, the reticle is filled with differently shaped things, and the one that fits your target tells you how far the object is. This too is a simplified description. Check out
http://www.springfield-armory.com for a description on how most rangefinding scopes work, and check out
http://www.swfa.com/mildot/index.html for a description of the mil-dot concept.
