This is a great article that I found online about MOA:

MOA, or Minute of Angle, is a common measurement for describing various characteristics in rifle scopes. It is most generally used to illustrate the click value for windage and elevation turrets on rifle scopes. The click worth is how much the reticle moves per one click on the turret. Values from 1/8 to 1 MOA are generally experienced. For dot sights, MOA is usually used to describe the dot size and, within the instance of the circle-dot reticle, the circle size as well.

As a general rule of thumb, we say that 1 MOA equals 1 inch at 100 yards. By extension, that would be a couple of inches at 200 yards, 3 inches at 300 yards, etc. The smaller the click worth, the a lot more precise the windage and elevation modifications will be on your rifle scope. This precision could be essential during competitive shooting and for long-range shots. Furthermore, understanding the circle size in circle dot reticles helps make rifle scopes useful for rangefinding.

In our information of MOA, we'll start having a circle. Naturally, a circle is a geometric figure where all points are equidistant from a single point, known as the center. The distance from the center to the circle may be the radius and a segment of the entire circle is called an arc. The circle is divided into 360 degrees and every degree is divided into 60 minutes. Returning to our rifle scopes for a moment, one MOA is 1/60th of one degree.

For our illustration, we'll use a circle having a radius of one. Let's say it's one yard. If you paid attention in that geometry class, you'll remember that a circle with a radius of one is known as a Unit Circle. Utilizing a unit circle makes all the math easy. The circumference of the circle is decided by the formula, a couple of(pi)r, exactly where r is the radius and pi may be the constant worth 3.14. Pi is really one of those infinite decimals that keeps going on and on but we'll stop it at decimal a couple of points for our exercise.

So, our circumference is 2 times 3.14 times 1 or 6.28. Now we wish to determine the length of an arc determined by 1 MOA. So, 6.28/360 degrees is .0174 and we'll divide that by an additional 60 minutes to get .000291. Of course, we don't require rifle scopes to shoot anything one yard away, but we can calculate the length of an arc at 100 yards. To do so, we'll simply multiply the .000291 times 100 to get .0291. Easy math right? Now, to convert that to inches we'll multiply it by 36 since you can find 36 inches in a yard. So, .0291 * 36 = 1.0476

When peering through a rifle scope or red dot sight at a target 100 yards away, .0476 is a pretty insignificant amount so we merely rely on our rule of thumb that 1 MOA equals 1 inch at 100 yards.

Source: http://ezinearticles.com/?MOA-and-Adjusting-Your-Rifle-Scopes&id=3315444

Bushnell Elite Tactical 1x32 3 MOA Red Dot Box ET1X32$349.00 Time Remaining: 3h 4m Buy It Now for only: $349.00 |
Trijicon 15x16S ACOG Amber Ring 2 MOA Dot 400138$1,032.00 Time Remaining: 4h 14m Buy It Now for only: $1,032.00 |
Redfield Battlezone 3 9x42 Rifle Scope TAC MOA Reticle Matte 91761 w Mounts$177.77 Time Remaining: 4h 29m Buy It Now for only: $177.77 |

Trijicon 15x16S Compact ACOG Green Ring 2 MOA Dot 400140$1,032.00 Time Remaining: 4h 31m Buy It Now for only: $1,032.00 |
Burris Predator Quest 3 9x40 Riflescope Ballistic Plex E1 Reticle 1 4 MOA Camo$213.98 Time Remaining: 8h 49m Buy It Now for only: $213.98 |
Millett Long Range Scope 6 25x56mm 1 4 MOA Click Mil DotBar Reticle Matte$584.95 Time Remaining: 9h 12m Buy It Now for only: $584.95 |

Vortex Crossfire II 618x44 AO Riflescope V Brite Reticle MOA CF2 31029 NEW$234.00 Time Remaining: 9h 25m Buy It Now for only: $234.00 |
Burris XTR II Scope 4x 20x 50mm Illum SCR MOA Frnt Focal XT 100Mil 201043$1,035.67 Time Remaining: 9h 42m Buy It Now for only: $1,035.67 |
Vortex Spitfire 3x Prism Scope EBR 556B MOA Reticle SPR 1303$300.00 (21 Bids) Time Remaining: 10h 30m |

Nightforce C522 SHV 4 14x56mm 25 MOA Illum MOAR Reticle Rifle Scope Black$1,106.00 Time Remaining: 11h 7m Buy It Now for only: $1,106.00 |