About bullet drop.
We know that the bullet drops as it travels down range.
Ballistic calculators will tell you how much the bullet drops if you give them all the proper data to calculate it.
Our scope's line-of-sight is above the barrel by an inch or two.
Gravity starts acting on the bullet as soon as it leaves the barrel.
Therefore, we have to point the barrel up in order to reach our 100yd zero.
This is to account for both the bore to scope height as well as bullet's drop from the line of departure.
This will cause the bullet to cross the line-of-sight around 85-90yds on a typical 308 load and then drop back down to your exact aiming position at 100yds.

When you are turning the scope knob, you are moving the reticle around within the scope.
Since we use the reticle to POINT the gun, what we are really doing is changing where the barrel is pointing.
So, we are using the scope's knobs to change the ANGLE of the barrel in relation to the line-of-sight.
This is really important to grasp because everything references back to the angle that you launch the bullet.

In order to hit targets that are farther away, you have to point the barrel up some more.
The graphic below calls the barrel angle the "line of departure".



With a 200yd zero, the bullet is probably crossing the line-of-sight around 50yds.

Zero it.
Before you can really use your scope, you need to zero it.
When I mount a scope on a rifle, I bore sight it.
To do this, put the rifle in a solid rest where you have at least a 50yd view.
Remove the bolt so you can look down the barrel and see something specific such as a target or perhaps a street sign or the tire of a parked car if you do this at home.
Adjust the position of the rifle so that the object is well centered in the bore and the rifle is stable so it will not move from that position.
Now look through the scope without disturbing the rifle's position.
Adjust your scope knobs until the crosshair is centered on the same object as you have centered in the bore.
Re-check the bore to be sure the barrel is still pointing at the same object and then final check that the crosshair is also adjusted to the same object.
Now you are ready for live fire.
You should be on paper at 50yds so fire 1 round at that distance and make a correction on the scope knobs if you are more than 4 inches off.
Now move out to a 100yd target and fire another round.
You should be within a few inches.
Make your final correction so that when you fire a round, it lands exactly where you were aiming at 100yds.
Now zero your scope knobs.
Every scope manufacturer has a different way of doing this, but most rely on setscrews that you loosen and slip the outer knob, or a center screw that you remove and lift the knob off, then replace in the proper zero orientation.
It does not matter how you do it as long as you end up with the "0" at the proper index park when you are at your 100yd zero setting.
Most scopes have several horizontal index lines to help you find what rotation of the knob you are on.
I recommend taking a black sharpie marker and blacking out all the lower lines, but leave just one line visible below the knob.



This way you will always be able to return to your proper zero position.
Later on, you will build your bullet drop data card from your 100yd zero.
You will record how much additional angle you had to add to get the bullet to hit the aiming point at further distances.
That's why a proper 100yd zero is so important as well as being able to get back to the proper 100yd zero.
It's not uncommon to become "lost in the knob" and lose which rotation you are on.
This put you WAY off your 100yd zero, usually 10" or more at 100yds.

Second Focal Plane vs. First Focal Plane.
There are two types of variable power scopes out there.
Most scopes that you will encounter are 2nd focal plane.
This means that the reticle is placed in the 2nd focal plane within the scope's optical design.
2nd focal plane reticles will not change in size when you adjust the magnification of the scope.
2nd focal place reticles are only accurate at ONE POWER SETTING.
In most cases, the reticle is accurate at the highest power setting that the scope offers.
You must know what power setting your scope is accurate at and USE THAT SETTING for all measurements.
Alternatively, we have First Focal Plane scopes.
First Focal Plane scopes have the reticle placed in the first focal plane of the scope's optical design.
This means that the reticle appears to change size within the scope as you adjust the magnification.
The reticle is actually staying the SAME SIZE in relation to the target you are observing.
This is a huge benefit because the reticle is now accurate at ALL POWERS.
This allows you to hold-off or MIL targets at any power without having to be concerned that you are on the proper scope setting.
The major advantage of first focal plane scopes is being able to use the reticle at lower powers where you have more field-of-view.
This increased field-of-view enables you to move quickly from one target to the next.
The major disadvantage of first focal plane scopes is that the crosshairs often become very thick at higher powers.
The thick crosshair can obscure small targets, which makes it harder to be precise on very small targets.
The advantage of second focal plane scopes is that the reticle size does not change so you can be more precise on small targets.
The major disadvantage of second focal plane scopes is that they must be used on a specific magnification setting in order for the reticle to be accurate.
This usually means that you will not have as much field-of-view as you will be on a higher power setting.
Fixed power scopes all behave the same as a first focal plane because they do not have any magnification adjustment.

_________________
God shoots a Remington 700, drives a Chevy Truck and carries a Glock 19.
I don't do any of those things.

About the markings on the knob and the reticle.
There are two common measurement systems in use on scope knobs as well as some uncommon ones which I will ignore for now.
The most common system is Minutes of Angle, also known as MOA.
The other common system is Milradians, also known as MILS.
Both of these are angular measurments.
One minute of angle is 1/60th of a degree.
Of course there are 360 degrees in a full circle.
So there are 21,600 MOA in a full circle.
One milradian is 1/1000th of a radian and there are 6.283 radians in a full circle.
There are 6,283 MILS in a full circle.
What's important to know is that 1 MOA equates to 1.047" per 100yds and this is commonly rounded to 1" per 100yds.
So, if you are shooting at 100yds, a 1 MOA change is basically 1" on target.
What's important to know about MILS is that 1 MIL is equal to 1/1000th of your distance to target.
So, if you are shooting at 100yds (3600 inches), a 1 mil change is 3.6" on target.
You can convert back and forth between MOA and MILS using the factor of 3.437.
1 MOA = 0.291 MILS or 1 MIL = 3.437 MOA.
I commonly round the factor to 3.5 for easy math in your head.
The following diagram will give you an idea what the angle looks like.



You can see that MILS are much larger units of angle than MOA.
There is another whole thread here that you can read on MOA vs. MILS as it relates to reticles:
viewtopic.php?f=10&t=222

Determine the range with the reticle.
Since we now know that the markings in the reticle have a known dimension, we can use that scale to determine the distance to the target.
In order to do this, you must know the size of the target though.
There are many good online tutorials covering this so I will just suggest that you check out these links:

Insert a list of mildot links here.

In addition to using the formula, there is also a tool called a mildot master.
This is a simple sliderule that allows you to quickly figure the distance without doing any math.
Check out the website and especially the online demo here:
There are also iphone and pda apps that will do the calculation for you.
The most important part of "milling" (measuring) a target is to get the measurement as accurate as possible because small differences in measurement make huge differences in the calculated distance to target.
I recommend that everyone keeps a mildot master in their kit as it can be used for both MIL reticles and MOA reticles.
To use the mildot master with an MOA scope, just move over to the right side and use your target size as the "bullet drop" dimension.

Calling corrections and Hold-over vs. Dialing.
It's possible (and recommended) to use your reticle to view your bullet's impact and then make a correction to make the next shot a better hit.
Depending on the distance, you may not get the reticle back on target before the bullet lands, but as long as you can see WHERE the bullet landed and make a mental picture, you can then put the reticle back on target and measure to where the shot landed by recalling the mental picture.
For example, let's say you fire a round and you observe the impact is 1 MIL to the left.
This is probably because you did not judge the wind well, or the wind changed after you made your judgment.
You can either hold the crosshair 1 MIL to the right and send another round right away, or you can dial a 1 MIL right correction and fire again with the same hold as before.
The advantage of holding-off for wind with the reticle is that you can watch the wind conditions and make small changes quickly just before you fire.
The advantage of dialing is that it lets you have a more precise sight picture, but it's slower than holding-off using the reticle.
The same rules apply to elevation corrections.
If you (or your spotter) calls a 1/2 MIL elevation correction, you can hold-off if you are in a hurry or dial it if you have the time.

_________________
God shoots a Remington 700, drives a Chevy Truck and carries a Glock 19.
I don't do any of those things.

Parallax
Parallax is an error caused by your scope's focus not being adjusted to match the target distance.
You must adjust the parallax/focus before you can expect precise bullet placement.
Lower cost scopes may not have parallax adjustments.
These are usually set for 100yds.
Some scopes will have actual range markings on the focus knob, but most will make you figure it out by eye.

To adjust the parallax, start by focusing the image as you look through the scope.
Then get the rifle on a solid rest with the reticle on-target.
Now move your eye up and down or left and right WITHOUT disturbing the rifle's aim.
If you see the reticle moving around on-target, that is a parallax error you are seing.
Continue to make small adjustments to the focus until the reticle does not move on target.
Now you are ready to make a precise shot.

Some scopes may go slightly out of focus when the parallax is set properly.
This is not desirable, but it's more important to have the reticle parallax-free than perfect focus.
Go with perfect parallax over focus every time.
On my nightforces, I usually find that if I find perfect focus, I need to roll the top of the knob about 1/16" forward to find perfect parallax.

The more consistent your cheekweld on the stock is, the less parallax error you will see.
This is one reason that a proper consistent cheekweld is so important.
A scope with no parallax adjustment will require you to be much more diligent in your position.
The more finely tuned you get the parallax, the more sloppy your position can be.

Tracking Test
You should test that your scope tracks properly.
This is also known as a "Box" test.

What you want to do is to go to the range and setup a nice clean LARGE piece of cardboard at exactly 100yds.
Make sure the cardboard is level left/right.
Setup a single target towards the bottom of the cardboard.

Verify that you are on a proper zero with the gun level left/right and fire a group on the target.
Mark the shots already on the target so you know which ones were already on that target.
Adjust your scope left 10moa or 3 mils.
Fire a couple rounds at the same target.
Adjust your scope up 15moa or 4 mils.
Fire a couple rounds at the same target.
Adjust your scope up another 15moa or 4 mils.
Fire a couple rounds at the same target.
Adjust your scope 20moa or 6 mils right.
Fire a couple rounds at the same target.
Adjust your scope back down 15moa or 4 mils.
Fire a couple rounds at the same target.
Adjust your scope back down another 15moa or 4 mils.
Fire a couple rounds at the same target.
Finally, adjust your scope back to the left 10moa or 3 mils.
Fire a couple rounds at the same target and check to see that they landed within the original group that you shot.

If your scope is tracking correctly, you should end up with a rectangle on the target frame that's 20.9" wide and 31.4" tall if you are shooting a scope with MOA knobs.
If your scope is tracking correctly, you should end up with a rectangle on the target frame that's 21.6" wide and 28.8" tall if you are shooting a scope with MIL knobs.

The rectangle should be formed perfectly square and level.
If the rectangle has good square corners, but the whole box is angled, then your scope is probably not mounted straight.
If the rectangle looks more like a parallelogram, then you have a scope problem.
If the rectangle does not have square corners, you have a scope problem.

_________________
God shoots a Remington 700, drives a Chevy Truck and carries a Glock 19.
I don't do any of those things.