Drop Units

All of the JBM trajectory calculators allow the user to select the drop, windage and lead (if applicable) units. Drop units are either a length such as inches or centimeters or an angle such as milliradians (mils), inches/100 yards, cm/100 meters, etc. The length is a user input value. The key is to input a value and units that makes sense for your sights or scope.

Length Units

For a typical trajectory calculation with drop in inches, the user would enter 1.0 for the value and select inches for the units. This would calculate a trajectory with units as multiples of inches.

Although it's not used often, one enter a non-unity (other than 1.0) value and calculate trajectories in multiples of inches. For example, entering "0.5" and units "inches" would calculate a trajectory with drop values in multiples of half inches. The numbers listed for drop are then the number of "half inches". Again, not often used.

Example 1

For drop units of inches and minutes of angle, enter 1.0 in the Column 1 Units box and select "in" for the units and enter 1.0 in the Column 2 Units box and select "MOA" for the units.

Example 2

For drop units of mils and centimeters, enter 1.0 in the Column 1 Units box and select "mil" for the units and enter 1.0 in the Column 2 Units box and select "cm" for the units.

Angular Units

With angular units (as used by all rifle scopes), the JBM calculations allow for accurate setting of scope drop and windage units. Units allowed are milliradians (mil), minutes of angle (MOA), inches per 100 yards and centimeters per 100 meters. One can measure the actual drop units (by shooting a target, moving the scope turret, shooting again and measuring the difference) or use the published values of the scope and enter them directly.

The angular units of inches per hundred yards and centimeters per 100 meters correspond to an angular change of 1 inch (or centimeter) at 100 yards (or meters). Note that at 200 yards, 1 inch/100 yards is 2 inches. At 300 yards, it is 3 inches and so on.

Example 3

Your scope has windage and drop values of one inch at 100 yards. To calculate a trajectory in scope "clicks", enter 1.0 for the Column 1 Units box and select in/100 yards as the units.

Example 4

Your scope has windage and drop values of 0.25 minutes of angle. To calculate a trajectory in scope "clicks", enter 0.25 for the Column 1 Units box and select "MOA" as the units.

Example 5

Your scope has windage and drop values of 0.5 inches at 50 yards. To calculate a trajectory in scope "clicks", enter 1.00 for the Column 1 Units box and select in/100 yards as the units. Note that you cannot enter 0.5 since the units are at 100 yards. You have to divide the 100 by the scope click range (100/50 = 2) and multiply by the scope click value (2.0*0.5 = 1.0) and enter that value.

Output

When the entered value is not "1.0", the column header will show "Clicks". The trajectory calculators will output a line in the "Input Data" section (above the trajectory table) that shows the actual units.