Re: MOA
Montour summed it up pretty well already, but here's some food for thought.
The angle you're dealing with that this relates to is the boreline (BL) to line of sight (LOS) angle, refered to in minutes of angle (MOA).
The BL extends out the bore in a "straight" line to infinity, first off.
LOS extends from the crosshair to infinity in a "straight" line also.
Bullet DROP is measured from your imaginary BL.
Bullet PATH is measured from your imaginary LOS.
DROP is measured in a PLUMB line from the BL, PATH is not, but at a right angle to LOS.
As soon as a bullet leaves the barrel, it immediately begins to fall from the BL because of gravity. How much it falls depends on the time of flight (TOF).
Side note 
A bullets BC determines its rate of velocity decay, thus effects its TOF. A higher BC bullet will have a shorter TOF if both are fired at the same MV, temperature and barometric pressure being the same also.
With LOS intersecting the bull at 100 yards, and if your BL is actually "paralell" to your LOS, the bullets point of impact (POI) will be low.
The amount it will fall below this point of aim (POA) is equal to the DROP at 100 yards, added to your scope's height.
For this example, your scope height will be 1.0", and your drop from BL will be 2.0". So, in the above "parellel" (BL and LOS)situation, your POI would be 3" lower than POA.
Bullet PATH would be 3". (From LOS)
Bullet DROP would be 2". (From BL)
In order to move POI up, you need to dial the elevation turret so the LOS moves toward, and intersects the BL at POA, or 100 yards.
A scopes turret adjustments are designed and calibrated so each click (line) moves the crosshair .25 MOA (a hair more than .25") at "100 yards", most of them are anyway. 4 clicks, or lines on the turret will equal 1 MOA, or just a hair over 1" (1.0472" exactly).
In our example, you would dial 3 MOA into the elevation turret to compensate for 1" of scope height and 2" of DROP at 100 yards. With LOS back on the bull, the BL is now effectively 2" above our LOS at 100 yards. The bullet drops 2" from the BL at 100 yards, and now 2" below the BL is our POA... drop is now compensated for perfectly.
We call bullet PATH, the drop below LOS beyond our zero range, or hight above LOS before or zero range. Trajectory + LOS.
So, at 200 yards, our bullet PATH is say, 4" below our LOS.
What do we do to compensate for this if it's beyond 100 yards? Dial 4 more MOA in? NO, only at 100 yards 1 MOA equals 1".
Picture this; your BL intersecting 2" above your LOS at 100 yards, both straight lines, and begining 1" apart at the rifle. So, if you dial 1 MOA into the turret now, at 100 yards your POI will change 1 inch, BUT at 200 yards it would change POI 2 inches, 300 yards it would change POI 3", at 650 yards it would change POI about 6.5", so on and so forth.
The angle remains the same, 1 MOA, but the range beyond that is 2, 3, or 6.5 times "that" which 1 MOA is equal to nearly 1" (100yds). Remember that bullet DROP is measured from the BL, and the BL at any given angle is 2, 3, or 6.5 times higher than your LOS depending on the distance away from your 100 yard zero.
If your target is at 1000 yards, it's 10 times farther than 100 yards. So, to move the POI 10" you'd dial in 1 MOA. The same 1 MOA change in adjustment is moving the BL 1" higher at 100 yards, but 10" higher at 1000 yards. Draw the angles out on a piece of paper to scale and you'll see it easily. POI would actually change 10.47" exactly, because 1 MOA changes POI at 100 yards 1.047", a hair more than 1". Thus 1.047 x 10 = 10.47".
If you know the bullet path is 57" at 600 yards from 100 yard zero, and you want to dial it in and rezero it for 600 yards instead of holding 57" over your bull, you know it's 6 times as far, so you take 57 and divide it by 6 to come up with the MOA to dail, which is 9.5 MOA.
To go one step further, you record the MOA difference between each 50 or 100 yard increment on your drop chart in order to move from one zero range to another quickly. Say for instance you just dropped a Coyote at that 600 yards, but the next one stops and gives you a shot at 700. If you know there's 2.75 MOA more needed from 600 to 700, you can just crank that on and fire.
If a target's at 687 yards, do yourself a favor and move the decimal over two places and divide this inches by 6.87 to compensate for drop, or to move the bullet impact a certain number of inches left or right...
Think in terms of MOA. Your drops, wind drift etc should all be recorded in MOA.
Clear as mudd?
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__________________
Brent Moffitt
