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

Weaver K Series 4x38 Riflescope Dual X Reticle 1 4 MOA Matte Black Finish 849415$160.83 Time Remaining: 33m Buy It Now for only: $160.83 |
730132 Bushnell Trophy Red Dot Sight 32mm Illuminated Red and Green 3 MOA T Do$165.83 Time Remaining: 33m Buy It Now for only: $165.83 |
Vortex Diamondback HP 4 16x42 Riflescope with V Plex Reticle MOA DBK 10021$329.00 Time Remaining: 2h 37m Buy It Now for only: $329.00 |

Burris 275x20mm Scout Rifle Scope Heavy Plex Reticle 1 05 MOA Matte Black$233.90 Time Remaining: 3h 34m Buy It Now for only: $233.90 |
31004 Sightron SIH Series 3 9x40 Riflescope HHR Reticle 1 Tube 1 4 MOA Matte Bl$119.66 Time Remaining: 3h 34m Buy It Now for only: $119.66 |
Weaver Super Sla 20x50 Riflescope EBX 1 8 MOA Matte Black$640.57 Time Remaining: 3h 49m Buy It Now for only: $640.57 |

Bushnell AR Optics TRS 32 5 MOA Red Dot w Mount Riflescope$168.99 Time Remaining: 3h 50m Buy It Now for only: $168.99 |
MAG624X40 Tasco Target and Varmint Riflescope 6 24x40 30 30 Reticle 1 4 MOA Matt$86.17 Time Remaining: 4h 33m Buy It Now for only: $86.17 |
Vortex Diamondback 39x40 Riflescope V Plex Reticle MOA DBK M 01P$199.99 Time Remaining: 4h 47m Buy It Now for only: $199.99 |

Tasco World Class Riflescope 3 9x40mm VZR 500 Reticle 1 4 MOA Matte Black$66.48 Time Remaining: 5h 18m Buy It Now for only: $66.48 |