Here is my crack at it - I will add more as time allows...
Terms:
Reticle: the 'targeting' part of the optic. If you were to point the optic at a plain white wall, this is all you would see. The reticle may be a simple crosshair, a crosshair with intersecting lines or dots along its length, a single dot, or an elaborate display.
Reticles whether simple or complex can be illuminated for use in low light conditions; this means that the reticle will grow (usually but not always red). Illuminated reticles are generally an option, and not all models of optics have this ability; those that do either require batteries, use a natural light gathering and focusing system, or a mix of both.
Some range estimating reticles have a row of different diameter circles which you can place over a target of known size – the circle with the same diameter of your target will tell you the estimated range. Other reticles which have hash marks or dots can be used to estimate distance by dividing the target size by the number of hash marks/dots to give the range in hundreds of yards/meters. See MOA or MRAD for more info.
Zero: The distance at which your bullet will hit the aiming point. For example, a 100 yard zero means that my bullets will hit exactly where I am pointing when the distance to my target is 100 yards. Similarly, a 300 yard zero would mean that my bullets hit where I aim when my target is 300 yards; they would hit above where I am pointing if the target was closer, and below where I am aiming if the target is farther away.
FOV: Field of View. This refers to how wide of an area you can see while looking through the optical device. As magnification increases, your FOV will decrease. The wider the better; a bigger FOV allows you to locate your target quicker, and allows you to see more of the surroundings around your target.
FFP: First Focal Plain. While most reticles callibrated with MOA lines or Mildots need to be on their highest power to perform range estimation, a riflescope with FFP can range at any magnification. With a FFP riflescope the reticle changes size along with your target as you change magnification; it can be a tiny little crosshair in the center of your field of view or a large crosshair which spans the entire width of your field of view.
Eye Relief: The distance from the back of the optic to your eye when looking properly through the optic. Cheap scopes may have very little eye relief; couple that with a lot of recoil and you may end up with a black eye.
Parallax: When you move your eye from side to side, or up and down while looking through an optical device, you may see the reticle move around over the target. This would mean that if your eye was not positioned in exactly the same place from shot to shot, your bullets would land in different places accordingly. Many optics have adjustment to help eliminate parallax. When there is no parallax the reticle will point at the same place regardless of your eyes position.
MOA /
Minute
Of
Angle: This is an angle which equals 1/60 of a degree. This works out to be 1.04719... inches at 100 yards. Because this is so close to 1, many shooters round 1 MOA down to 1" at 100 yards. This keeps things very simple, but if you get into more precision shooting you will need to start using a few more decimal places, perhaps 1.047. Think of MOA as a circle not a line; picture a giant cone protruding from the muzzle of your rifle; the cone represents the area your bullets will spread out in as they travel. If the cone measures 1" wide at 100 yards it will be 2" wide at 200 yards, 5" wide at 5 yards, 12" wide at 1200 yards, and so on.
1 MOA at 100yards is 1.047" or roughly 1"
1 MOA at 200yards is 2.094" or roughly 2"
1 MOA at 300yards is 3.141" or roughly 3"
1 MOA at 400yards is 4.188" or roughly 4"
1 MOA at 1000yards is 10.47" or roughly 10"...
MOA is commonly used as a measure of a rifle or shooters level of precision; for example if I fired 5 shots at a 100 yard target and the holes formed a group which measured 1.05" at its widest, I could then say I shot exactly 1MOA. If my 5 shot group measured 2" wide at 400 yards I would have shot just under 1/2MOA since 1 MOA at 400 yards is 4.188" (2"/4.188MOA = 0.48).
The MOA concept is useful in shooting at distances beyond where your rifle is zeroed. Hopefully the following scenario will help you grasp how to use this concept (I will round 1MOA to 1" at 100yards for ease of learning):
-Your rifle is zeroed at 100yards. You have a scope with 1/4MOA adjustments. You shoot at a 300yard target and notice your bullets form a group roughly 12" below where you were aiming. How many clicks do you adjust your optics so that you hit exactly where you are aiming?
First you have to convert the distance you were off target from inches into MOA, so:
12" (distance off target) divided by 3" (MOA at 300 yards) = 4MOA
Saying you were 4MOA off is the same as saying you were 12"off at 300yards.
Now divide how far off you were (4MOA) by your scopes adjustment per click (1/4MOA or 0.25MOA)
4MOA / 0.25MOA per click = 8 clicks
Turn your elevation adjustment up 8 clicks and your bullets will now hit exactly where you are pointing.
MRAD / Millradian: This is the metric equivalent to MOA and is used similarly.
Drop Chart: A quick reference chart which shows how much to adjust your optics to shoot at a given distance. A simple drop chart might look like:
Distance – MOA Come-up – Drop in inches
100 – 0 – 0”
200 – 1.5 – 3.2”
300 – 3.5 – 10.9”
400 – 5.5 – 23.6”
500 – 8.0 – 41.7”
More elaborate drop charts might include information on adjustment for wind drift at different speeds or lead distances for moving targets. Drop charts can be created by either entering data into a ballistics calculator or by trial and error shooting.
Come Up: This is a term which refers to how many MOA or MRAD you need your adjust your elevation to hit your target at a given distance. For example, a rifle zeroed at 100 yards which drops 41.9” at 500 yards would have an 8MOA come-up at 500 yards. See MOA to learn how to calculate this.
BDC: Stands for Bullet Drop Compensator. This is a form of reticle which estimates where you should aim at a given distance to compensate for how much the bullet will have fallen over the course of its flight. The aiming points in a BDC reticle are calculated mathematically based on a specific set of data (calliber, bullet weight, velocity, BC, etc.). Because it is unlikely that you will ever be able to exactly match all of these mathematical variables, a BDC reticle will likely only be a good guess, and not exact.
Lapping: Some bases or base/ring combinations are not manufactured as precisely as they could be. Lapping is a process which shapes the bottom mounted rings, by sanding away excess material, so that the scope will be perfectly alligned once secured in the rings. The top rings do not need to be shaped because it is the lower rings which the scope rests in - the upper half of the rings just keep the scope in place. Under certain circumstances lapping can be important, but if overdone you can end up with loose rings aroung your scope.
Optics Classes:
There are several designs of optics. Here are some of the common ones:
Riflescope: Basically a long tube that magnifies what you are looking at. Riflescopes along with other types of optics come in 2 forms – fixed power (ie 4X, 10X, etc.) or variable (ie. 3-9X 5.5-22X) power. Riflescopes are generally referred to in the following format:
Magnification power X diameter of lens (measured in mm)
For example, a 10X50 scope would magnify what you are looking at to 10 times its natural size, and would have a 50mm lens.
A 3-9X40 scope would allow you to adjust the magnification from 3 to 9 times its natural size, and would have a 40mm lens.
As the lens diameter increases the optical device can collect more light giving you a brighter picture. A large diameter lens can be especially useful in low light conditions, such as dawn or dusk.
Riflescopes will have at least two knobs or turrets. One will be for adjusting elevation (up and down), the other is for adjusting windage (left-right). Some riflescopes may also have adjustment for parallex or clarity.
All optics will need to be zeroed by adjusting the windage/elevation so that the reticle points at the right place. To adjust elevation or windage on riflescopes simply unscrew the caps, turn the dial the correct direction (will be marked), and replace the cap. Scopes with target turrets are adjusted by simply rotating the entire turret - once your rifle is zeroed you loosen a set screw on the cover which allows the 'scale' to rotate freely without adjusting the reticle; you then set the scale so to zero and then retighten the set screw. Now you can rotate the turrets and know where to return to.
Reflex Sights: Are generally found in non-magnified (1X) or low magnification (up to ~4X) formats. They work much like an overhead projector in a theatre or a heads up display in a fighter jet, by projecting the reticle onto the glass so that you can see it with your eye. Reflex sights are well suited for rapid, close range, both eyes open shooting.
Things to keep in mind when buying optics:
- Your optic will only be as good at the mount it sits on – a cheap SKS or M14 mount will be terribly inaccurate, even with a top of the line scope. Don’t cheap out on the base!
- The saying that “you get what you pay for” is also especially true with optics. Will a $100 Bushnel Scope work – yes, but it will have issues. Will a $2000 Nightforce riflescope work much better - yes, it will positively hold its zero, unlike a $100 scope its 1/4MOA adjustments will actually be 1/4MOA, it will give a much brighter picture and wider FOV… Find a balance that fits your realistic expectations and budget.
- Carefully consider your intended usage, with special emphasis on what range most of your shooting will take place. If you are buying optics for a short barreled AR15, what are you going to be shooting with it? Most likely you will want optics suitable for rapid target acquisition at close range. A reflux / red dot sight will be a likely candidate. If you are buying optics for a long range target rifle you will of course want something suitable for longer ranges, perhaps an expensive US Optics riflescope. More magnification will help you see farther distances but will make it slower to aquire your targets and make followup shots. If you have used variable power optics before, do you always leave it on a given power setting – perhaps a fixed power optic is in order?
- When looking for a tactical scope with MOA/MRAD adjustments be aware that many scopes may have MOA adjustments but a MRAD calibrated reticle – why, don’t ask me – I think its stupid but thats just how it is. Thankfully some scopes such as Nightforce have MOA adjustments and MOA calibrated reticles. Do your homework and decide if this is an issue for you or not.
- When looking for optics for an extreme long range rifle consider the balance between magnification and internal elevation adjustment. Higher magnification makes it easier to see your target, but only up to a certain point. Once you go so high you will start amplifying mirage. Of course you can always dial down a 12-42X scope to 22X to eliminate mirage, but you can’t dial a 5.5-22X scope up to 42X. But also remember how far you want to shoot – generally riflescopes with higher magnification have less internal elevation adjustment, which limits the maximum range you can shoot to. For most people this will never be a problem, but look to the future.
- Target turrets and calibrated reticles (mil dot or MOA hash marks) make jumping from distance to distance much easier. If you are looking for target optics keep this in mind – this will likely not be an issue if you are looking for bush hunting scopes.
-If you have a budget, do you really need that ‘night vision compatible’ feature? Sure its cool, but are you seriously ever going to use it? Of course if you have the money, why not?
How to mount a riflescope:
First decide if you are going to lap. I personally buy high end stuff and never lap, so I will leave it to someone else to explain lapping; there are already other threads on this topic. Here is just one:
http://www.canadiangunnutz.com/forum/showthread.php?t=474426 Always follow the directions! There are many methods, and many styles of bases / rings. Here is one method, assuming you are not lapping, and are using TPS rings:
1.) Make sure your rail / base is attached properly. If your base is loose your optics will be too. This means you will have trouble hitting what you aim at. Assuming you have predrilled holes in the receiver and base which match up, tighten the base screws to somewhere in the neighborhood of 21 in/lbs (NOT ft/lbs which would be way to tight). Use locktite – you don’t want the base coming loose!
2.) Place the rings on the scope and start all the screws, but do not tighten them yet.
3.) Set the scope with half attached rings as a unit onto the base, but still do not tighten anything. Move the scope back and forth inside the rings, or move the rings from slot to slot on the base; you are trying to find the most comfortable scope to eye distance which gives you the best sight picture. Once you have found the correct distance go to the next step.
4.) Set the scope to its lowest magnification power and while keeping the rifle perfectly vertical aim at a weight on a string. Attaching a bipod will make this job very easy. Rotate the scope inside the rings so that the vertical part of the crosshairs is parallel to the string. Be careful not to let the scope slide back or forth in the rings, if it does slip then you will need to repeat the previous step. Now tighten down the screws which connect the top and bottoms half of the rings. Follow the manufacturer’s directions, but if there are none, then 21 in/lbs should be a good estimate (again, be careful not to confuse with ft/lbs).
5.) Now, while keeping the rings in their slots on the base, slide the scope/ring unit as far forward as it will go (this will only be a hair). This will prevent the scope from slipping forward under recoil. Then tighten the nuts on the bottom of the rings onto the base. Again check the directions, but 65in/lbs (NOT ft/lbs) should be appropriate.
6.) Stop and stare at your new toy, admiring how great it looks before proceeding to zero the rifle.
***NOTE: in/lbs are not the same as ft/lbs – everything here is measured in in/lbs – if you tighten the screws/nuts to the same unit of ft/lbs you will wreck your scope!***
How to zero your optics:
As you get more skilled you can start at a further distance or jump longer distances as you go, but if you follow this procedure you will never go wrong. Use the largest target you can find; placing a chunk of 4’ rolled packing paper behind your target will make life very easy since you will be able to easily find your bullet holes. If you are good it should not take you more than 10 rounds to get your zero but if you are unsure of your abilities take at least 30 rounds. Take an extra target or two in case you get too many holes in the target that it starts to confuse you. Assuming you want a 100 yard zero:
1.) Set up a target at approximately 7 yards and fire one shot at it, then mark the hole on the target so you will always be able to tell which hole was your last shot.
2.) Adjust the elevation and windage dials on your optics the appropriate number of clicks so that you will hit the center of the target, fire one more shot which will now hit the center of the target if you have adjusted the optics properly. If your shot is not very close to the center of the target, readjust the scope as needed and fire another shot – do not proceed to the next step until you hit very close to the center of the target. Mark all bullet holes in the target as you go.
3.) Set up a target at approximately 20 yards and repeat the previous step. Fire one shot, adjust the optics until you are hitting very close to the center of the target - proceed to the next step only once you hit the near the center of the target.
4.) Repeat the same procedure at 40 yards, then, 60 yards, then 80 yards.
5.) Once you have hit the center of the 80 yard target go to 100 yards but instead of firing a single shot fire a 3-5 round group. If necessary readjust the optics and fire another group. Repeat as needed until the group is centering on the point you are aiming at.
Once your rifle is properly zeroed it should stay there, however some people like to reconfirm that they are zeroed before a special event such as a hunting trip or a competition.
Really really lost. 
