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Doyle · September 9, 2012, 01:42 PM

Google will give you more complete answers but here is the simple version.

MOA - Minute of Angle. It is a measurement of a portion of a circle. A circle is divided into 360 degrees. Each degree is divided into 60 subparts called minutes. One MOA is one of those minutes. In shooting, MOA gets translated into a linear distance. One MOA worth of angle difference, when measured at 100 yds is pretty close to 1 inch (not exactly but close enough for our discussion here). At 200 yds, that one MOA becomes two inches. Etc. With scopes, the ability to adjust the crosshairs is measured in MOA. A 1/4 MOA adjustable scope means each click of the scope adjustment will result in a POI (point of impact) change of 1/4 inch. In a red dot scope, there is also a MOA measurement for how large an area the dot covers. A 5-MOA dot will cover an area of 5" at 100 yds.

Parallax - this one is a bit harder. Think of the relationship of the crosshairs to your target as you are looking down the scope with your eye perfectly centered in line with the crosshairs. Now, imagine moving your eye slightly off that line of sight. Parallax is the movement of the crosshairs in relation to the target as your eye shifts off the centerline. Scopes are designed to be "parallax free" (i.e. no movement) at a certain distance. Scopes designed for a rimfire (i.e. fairly short range) are typically set at 50 yds. Scopes that are designed for centerfire calibers are either set to be parallax free at 100 yds or they are user-adjustable. The poorer the quality the scope, the more noticable the parallax change will be as you get away from that parallax-free distance.

September 9, 2012, 01:42 PM	#2
Doyle Senior Member Join Date: June 20, 2007 Location: Rainbow City, Alabama Posts: 7,167	Google will give you more complete answers but here is the simple version. MOA - Minute of Angle. It is a measurement of a portion of a circle. A circle is divided into 360 degrees. Each degree is divided into 60 subparts called minutes. One MOA is one of those minutes. In shooting, MOA gets translated into a linear distance. One MOA worth of angle difference, when measured at 100 yds is pretty close to 1 inch (not exactly but close enough for our discussion here). At 200 yds, that one MOA becomes two inches. Etc. With scopes, the ability to adjust the crosshairs is measured in MOA. A 1/4 MOA adjustable scope means each click of the scope adjustment will result in a POI (point of impact) change of 1/4 inch. In a red dot scope, there is also a MOA measurement for how large an area the dot covers. A 5-MOA dot will cover an area of 5" at 100 yds. Parallax - this one is a bit harder. Think of the relationship of the crosshairs to your target as you are looking down the scope with your eye perfectly centered in line with the crosshairs. Now, imagine moving your eye slightly off that line of sight. Parallax is the movement of the crosshairs in relation to the target as your eye shifts off the centerline. Scopes are designed to be "parallax free" (i.e. no movement) at a certain distance. Scopes designed for a rimfire (i.e. fairly short range) are typically set at 50 yds. Scopes that are designed for centerfire calibers are either set to be parallax free at 100 yds or they are user-adjustable. The poorer the quality the scope, the more noticable the parallax change will be as you get away from that parallax-free distance.