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=222Determine 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.