One of the most important pieces of fundamental shooting is learning how to zero your rifle mathematically.
What is MOA?
Most of the time when you are talking about angles we use degrees. Well 1 degree at 100y is 60". That's too big an increment to work with for most sighting systems. 'Minute" is french for 1/60th. So a Minute Of Angle is 1/60th of 1 degree.
What's the formula for calculating 1MOA?
1 MOA is 1 inch *PER* 100 yards (okay it's actually 1.047" but for all intense and purposes 1" works)
1MOA is 1" @100y
1MOA is 2" @200y
1MOA is 5" @500y
1MOA is 3 3/4" @375y
1MOA is 1/2" @50y
1MOA is 1/4" @25y
So if you have a *sub* MOA gun that means it will group less than 1" @100y
Okay, fascinating but why do I care?
Well, once you understand MOA and it's relation to the size and location of your groups on your target you can now make precision sight adjustments.
Lets say you have a 5 shot group that measures approx. 1 1/2" @25y. That means you shot a 6MOA group (1MOA @25y = 1/4" - 1/4" goes into 1 1/2" 6 times = 6MOA group). Now lets say that group was 3" left and 1 3/4" low and you want to make a sight adjustment.
Step 1: Calculate how many inches you want to move. 3" right and 1 3/4" up
Step 2: Convert inches to Minutes. 1MOA= 1/4" at this distance and 1/4" goes into 3" 12 times or 12 Minutes. 1/4" goes into 1 3/4" 7 times or 7 Minutes
Step 3: Make the appropriate sight adjustment
This can be a little tricky. You really have to know your specific sighting system as to how many Clicks to put in per Minute. Generally speaking stock military sights (AR15, M1, M14) are 1 click per MOA. Most scopes are labeled under the cap rings 1/4" @100y (or 1/4 MOA = 4 clicks per MOA) or 1/8" @100y (or 1/8 MOA = 8 clicks per MOA) Red dots tend to be 1/2 MOA per click. It varies by sighting system.
Lets assume you have a 1/4" @100y scope, you would put 48 clicks (12 MOA*4 clicks per MOA) right and 28 clicks (7 MOA*4 clicks per MOA) up.
If you had a 1/2" @100y red dot, you would put in 24 clicks (12 MOA*2 clicks per MOA) right and 14 clicks (7 MOA*2 clicks per MOA) up.
Irons are even easier as they are frequently 1 to 1. 12 clicks right and 7 clicks up.
This works no matter what distance you are shooting at. Put your target up at 100y and if you shoot 3" left and 1 3/4" low then you need to make a 3MOA (1MOA @100y is 1") adjustment right and a 1 3/4 MOA adjustment up. 12 clicks and 7 clicks in our 1/4" scope example.
Put your target up at 75y and if you shoot 3" left and 1 3/4" low then you need to make a 4 MOA (1MOA @75y is 3/4") adjustment right and a 2 1/4MOA adjustment up. 16 clicks and 9 clicks in our 1/4" scope example.
If you don't know (or you want to verify) you can shoot a group at a known distance; say 25y. Then make a 10 click adjustment and shoot another group (keep your Point Of AIM the same). Measure the distance between the groups and calculate how many minutes you moved and divide by the number of clicks you put in. You will now know how many clicks per MOA.
If you want to know how good your sighting system is: Shoot a 5 shot group. Make a 10 MOA adjustment to the right. Keep your same POA and shoot another 5 shot group. Make a 10 MOA adjustment down. Keep your same POA and shoot another 5 shot group. Make a 10 MOA adjustment left. Keep your same POA and shoot another 5 shot group. Make a 10 MOA adjustment up. Keep your same POA and shoot another 5 shot group. If you end up on top of your first group then you have a very repeatable sighting system.
Okay, maybe I follow you but still; why do I care? Can't I just guess and check?
Yes you can.
If all you want to engage is paper targets at the same fixed distance then there really is no reason not to guess and check; however, if you want to take your shooting to the next level where you can fully understand bullet drop, wind calculations and engage targets at varying distances you *must* understand how your sighting system works. Inches converts to Minutes converts to Clicks.
But wait, what about milliradians?
3.6MOA per millirad.