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 Super Slam 2 10x50 Riflescope EBX 1 4 MOA Matte Black$502.85 Time Remaining: 2h 14m Buy It Now for only: $502.85 |
733960 Bushnell Trophy XLT 3 9x40 Riflescope Multi X Reticle 1 4 MOA 1 Fast Foc$123.22 Time Remaining: 3h 15m Buy It Now for only: $123.22 |
FF13014 Firefield Riflescope 25 10x40 Illuminated Mil Dot Reticle 1 4 MOA 5mW G$119.83 Time Remaining: 3h 44m Buy It Now for only: $119.83 |

Simmons44 Mag 3 10x44 Riflescope Truplex Reticle 1 Tube 1 4 MOA Black Finish$97.38 Time Remaining: 3h 44m Buy It Now for only: $97.38 |
200367 Burris Predator Quest 2 7x35 Riflescope Ballistic Plex E1 Reticle 1 2 MOA$193.72 Time Remaining: 3h 44m Buy It Now for only: $193.72 |
200320 Burris Fullfield E1 3 9x40 Riflescope Ballistic Plex E1 Reticle 1 4 MOA M$243.01 Time Remaining: 3h 44m Buy It Now for only: $243.01 |

Bushnell 6500 25 16x42 Riflescope DOA 600 Reticle 1 4 MOA Black 652164B$684.08 Time Remaining: 3h 45m Buy It Now for only: $684.08 |
BSA Optics Deer Hunter Riflescope 3 9x50 Standard Matte 1 025 MOA BSADH39X50$68.41 Time Remaining: 3h 58m Buy It Now for only: $68.41 |
NEW BURRIS SPEED DOT 135 SCOPE 1X35 MM 3 MOA DOT 300200$131.94 Time Remaining: 5h 45m Buy It Now for only: $131.94 |

AC11386 Barska Point Black 3 9x40 Riflescope 3G Reticle 1 4 MOA 223 Caliber BDC$143.20 Time Remaining: 5h 58m Buy It Now for only: $143.20 |