The Rifle Scope Ruler makes it easy to sight in a rifle with as little as one shot. Just place the appropriate target distance scale on the target after your first shot and get a direct readout of how many clicks to move the scope for windage (left or right), and then do the same for elevation (up or down). It's easy and accurate. The MOA version is designed for 1/4, 1/8, and 1/2 MOA turret scopes, and calibrated with scales for 25, 50, 100, and 200 yards. The MOA version is what most scopes sold in the U.S.A. will need to get sighted-in accurately. The MRAD version is designed for 0.1 Mil scopes, and is calibrated with scales for 25, 50, 100, & 200 yards, not meters. The Rifle Scope Ruler will satisfy the demands of most all shooters for use with modern rifles, muzzleloaders, pistol scopes, crossbow scopes, and air rifle scopes, from .177 pellets to the .50 BMG and beyond.