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This calculator is similar to the older Optimum Point Blank Zero (OPBZ) calculator. 
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Like the OBPZ calculator, when given Sight Height, MV, and pellet BC, OBPZSIR will calculate the Optimum Point-Blank Zero that will keep the pellet within the desired trajectory Vital Zone Radius over the longest possible shooting range. This unique zero maximizes the direct-aim (point-blank) capability of the weapon.
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However, OPBZSIR differs from OBPZ in that an additional feature (Sight in Range) is included to assist in those cases when only preset shooting ranges are available. This new feature is intended to assist zeroing of the sight when it isn't possible to set a target at the actual calculated OPBZ. The OPBZSIR calculator makes that unnecessary by calculating the Point of Impact Hold Under (HUSIR in inches) corresponding to any convenient Sight in Range.
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Positive HUSIR indicates the number of inches the pellet will strike ABOVE the point of aim at the specified SIR. This means that the elevation screw of the sight should be adjusted for a point of impact HUSIR inches above the point of aim. Doing so is exactly equivalent to adjusting for dead-on at the OPBZ. Alternatively when negative, HUSIR equals the number of inches the POI should be adjusted BELOW the POA to achieve the optimum OBPZ.
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Steve_in_NC
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