Nightforce Ranging Calculations
The Nightforce BR range of scopes and most of the NXS range have the reticle located in the second focal plane. This means the reticle stays the same size throughout the entire zoom range. The advantage is that the reticle does not grow bigger and smaller as the zoom is used as it does with a first focal plane reticle. The disadvantage is that the calibrated graduations in both the vertical and horizontal axis are only accurate at the 'R' (ranging) setting.
The 8-32 x 56BR and the 12-42 x 56BR have the 'R' setting at 22x which is quite useful in itself. If you are using for example a NPR2 reticle then the vertical graduations are set at 2 MOA and the horizontal at 5 MOA. All other magnification settings require the graduations to be recalculated.
If you use the reticle graduations to centre your shots during the firing of the sighting rounds, then you need to know accurately what correction is needed to bring your shots into the centre of the target. This means you need to know the MOA the graduations measure at each zoom setting.
This is what needs to be done to calculate a correction for a zoom change to 32x. The calculation is simply the Calibrated Power (22) divided by the Operating Power (32) then multiplied by the calibrated graduation value (2) for vertical graduations on a NPR2 reticle.
Then 22 / 32 = 0.6875 and 0.6875 x 2 = 1.375. So at 32x magnification the vertical graduations are 1.375 MOA as opposed to being 2 MOA at 22x magnification. The horizontal graduation is simply 0.6875 x 5 = 3.4375. The horizontal graduation at 32x becomes 3.4375 MOA as opposed to 5 MOA at 22x.
Round these numbers down to one decimal place and put them on a simple chart inside your ammo lid and you now have an accurate way of ranging at all zoom settings. Download the spreadsheet shown below using this hyperlink: Zoom Compensation Sheet.
If you are using a different reticle just substitute the calibrated MOA graduations and magnification at your reticle's 'R' setting.
Article by Ian Pavy (7-6-09)