We know that gravity works only on the vector pointing at the earth's center when shooting a bullet. When taking a shot at extreme, angle up or down, the point of impact will be higher than for a shot taken at the same straight line distance over level ground. Drop can be re-calculated for the adjusted distance (perpendicular to gravity vector) by using the trigonometry function called the cosine of the angle. Lots of you guys out there could explain that better and in far greater detail than I.
What most of us care about, though, is how to quickly determine the needed adjustment in the field.
This device calculates the cosine function by use of a rotating level attached to the side of the riflescope. The readout is "cosine of the angle" rather than just "angle" so all you have to do with the readout is multiply this decimal times the lasered straight line distance to the target along the steep angle. The mounting system for the Cosine Indicator is a Leupold QRW ring. http://www.snipertools.com/aci.htm
In my photo it is attached just behind another neat gadget. This green(?) object is basically a spirit level that tells you when the rifle is canted off to one side or the other. RCBS's ballistics software can compute a figure showing how far off to the side and downward will be your point of impact at long range if you cant the rifle at a different angle than that you used in establishing your drop table in the field. My software copy isn't working right now but you would be depressed to know how significant this can be --- hunched up sideways on a hillside somewhere trying to level your rifle from some awkward position.
Darryl: Thank you, of course I meant to say in my post above that the point of impact is higher when shooting at an angle and the point of aim should be lower. It is now corrected above.
[ 08-02-2001: Message edited by: Len Backus ]