I have 4 AR15's
That's a common question, with too many answers. It has too many answers because it's the wrong question. The question you should be asking is, "What's the best scope in my price range for my AR-15 doing a specific kind of shooting?" Let's start with that. Remember, no one scope is best for everything. Some manage to be good at several things, but the laws of physics and optics inevitably force compromises.[/size]
Shooting 100 yd targets?
Sniping at 600 meters?
Competing in 3-gun events?
Killing gophers on the ranch?
Taking on a five day hunting trek?
Taking into combat in a war zone?[/color]
Each of these is a dramatically different mission, and each has its own requirements. There are some general principles, though, that you can usually depend on. And, naturally, there are exceptions.
First: A more expensive scope is usually made better.
Made of better materials, built to more precise tolerances, with better optics. They are also usually better sealed against the elements, more durable, and often heavier.
Second: A larger objective lens -- the glass facing your target -- can gather more light.
This means a brighter sight picture for you, especially in low light conditions, and that can make the difference between identifying a target and losing a shot. Of course, that big lens adds to the weight. Glass is heavy.
Third: A larger tube allows more light transmission and usually enables more adjustment for windage and elevation.
But again, a 30mm tube will weigh more than a 1-inch (25.4mm) tube. More importantly, the cross-sectional area is about 39% greater, which means all the glass inside that tube will be at least 39% heavier, and probably more.
Fourth: There's really no reason to buy a scope that can outrange the ammo you're shooting.
Remember, even good .223 or 5.56 ammo is running out of kinetic energy at 600-700 meters. If you go to something like .300 Blackout, you might get another 100-200 meters of effective range from it. If you have an AR-10 (or other .308 weapon), 1000 meters is possible. Either way, your AR won't be making bullseyes at a mile like a Barrett .50 might in the right hands. Get a scope that can work at the maximum range you'll shoot, but remember all that adds size, weight, and cost - so don't buy more scope than you can use.
Fifth: Lower magnification means wider field of view, which means it's easier to acquire your target and keep on top of the situation around it.
Higher magnification means you can watch things farther away, and potentially more ease in observing or identifying your target once you've acquired it. While a variable power scope offers both options to some degree, it's optically more complex, which again means greater weight and cost.
You notice the pattern in the downsides of each of these:
