March offers two blank large dials for shooters to create their own Ballistic Drop Compensator (BDC). These blank BDCs replace the existing elevation knob on your March scope and let you create your own elevation adjustments to suit your load. Make any changes to your load and replace the tape on the dial quickly and easily. This model can be used with 1-4.5x, 1-10x, 2.5-25x, 1-8x, 3-24x, and 5-40x magnification March scopes.