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What is Hold Under?<br>
by Steve_in_NC(pneuguy)<br>
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In an ideal world and if not for gravity, it would only be necessary to point the muzzle at the target and fire. Because the pellet would exactly follow the boreline forever (assuming no deflecting wind and that the aim is true) a hit would be guaranteed regardless of range.
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In real life, of course, this doesn't happen. Gravity spoils the plan.
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The instant the pellet leaves the muzzle, gravity accelerates it downward. I.e., it starts to fall. Slowly at first, then faster, gravity's pull makes the trajectory curve away from the boreline. Therefore, in order to hit distant targets the trajectory's curve must be compensated by angling the bore line upward relative to the line of sight (i.e., elevating it) so that the pellet leaves the muzzle with the right vertical velocity to neatly cancel its drop on its flight to the target.
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A problem arises, however, when the line of sight is inclined -- as when shooting upward into trees or downward from an upstairs window. That's because only the part of drop that's perpendicular to the line of sight contributes to the curvature of the trajectory. This means that when the line of sight deviates significantly from the horizontal (regardless if up or down), the amount of boreline elevation that would be correct if shooting on the flat becomes too much and, unless reduced, will cause the pellet to overshoot and miss.
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The reduction in elevation necessary to compensate an inclined shot is the "holdunder" calculated by this application.
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To use the Holdunder Calculator, enter the line-of-sight range, muzzle velocity, pellet ballistic coefficient, and either the angle of inclination, the horizontal range, or the vertical range of the shot. The answer returned (expressed in inches and minutes of arc) is the amount to aim below the intended point of impact to compensate for the tilt of gravity's vector.
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