The nearby graph shows how to determine the height over bore, or sight height, of your AR15 / M4 to be used in a ballistic calculator.
Height over bore, or sight height, is the difference in height between the center of the barrel / bore and the center of your scope. The height over bore of an AR15 / M4 can be determined by simply adding your scope mount height, or ring center to base, to the distance between the top of upper receiver picatinny rail interface and the center of barrel extension. Thanks to standardization, on a standard mil-spec M4 / M16 upper receiver, the distance between the top of picatinny rail interface and the center of barrel extension is 1.24″. As such, the height over bore of your AR15 is simply: your scope mount height + 1.24″.
Below are some commonly used scope mount heights and their respective heigth over bores.
1.54″ + 1.24″ = 2.78″
1.70″ + 1.24″ = 2.94″
1.93″ + 1.24″ = 3.17″
