“The crosshairs are moving when adjusting the side focus or parallax knob”
See the video I made with my mobile phone. The quality is not very good. It’s a hell of a job to keep the camera steady and in the exit pupil. But you will see what happened when I adjust the parallax knob on my Big Nikko.
Link to the video: Video of the crosshairs moving on the Big Nikko
Last weekend I was preparing for an 100 meter smallbore and airgun match. It has been quite a while since I last shot at 100 meters. So I thought I would check the zero of my .22 LR Anschutz and the .22 Theoben Rapid. Both have a Nikko Stirling Diamond Sportsman 10-50×60 MKII scope on top (the nikko with the low green turrets). To check the zero at home I used an bore sighter / collimator from Leupold: the Leupold Zero Point.
It attaches to the muzzle with a magnet and when you look through the scope you see an image like this:
You can then adjust the scope until the crosshair / reticle matches the position on the grid you want. I usually use a setting I measured at a zeroed rifle scope combination and wrote down. But it also helps a lot to get a new scope on target when you just set it to the 8 or 16 inch setting at the grid for a 50 meter zero. For 100 meter zero I need to set it around 20 to 24 on the grid for both the .22LR Anschütz and the .22 airgun at 40 joule.
What I noticed with both the Big Nikko scopes is that the crosshair moves up and down when the side parallax (or side focus) knob is turned. And not by a little, but by 2 inch and 4 inch on the grid for two scopes respectively! The error was very consistent. I turned the focus knobs many times and every time the crosshairs moved up and down again the same distance. And they do this shift gradually: The crosshair keeps moving with the increasing movement of the focus knob.
The crosshairs both moved upwards when going to the larger distance setting on the side focus / parallax knob. For a focus setting from 10 meter to infinity the reticle moved four inches up on the grid for the one scope. This means a four inch movement of your point of impact at 100 yards. Or approximately two inch at 55 yards which is the long field target shooting distance. This is not a small error. As has big consequences when you want to use Chairgun or other ballistic software to make a sight table or scope tape for your Field Target rifle. It will never give a correct match between calculated and real world elevation settings. And because in field target you use a different focus / parallax setting at each distance you shoot, the crosshair will move as well.
The crosshairs in the Big Nikkos are moving up with increasing distance. This will mean that the elevation correction needs to be much larger than expected from ballistic software when going to a greater distance.
This does not mean that these Nikko scopes can not be used for proper FT shooting. When using elevation turret settings that were actually observed when shooting, these settings will still match. This is because the error is very repeatable. But when you want to use Chairgun or other software to make a sight table or turret tape for your field target rifle it will just not work. And that is how I usually use Chairgun: To make a table for every 2 meters when just zeroing the scope at 8, 20, 30, 40 and 50 meters.
An other error I noticed is that both scopes moved the crosshair when adjusting the magnification. About a half to 1 inch@100 yards up and down. This happens when moving the magnification, or zoom ring, from one direction to the other. And instantly when starting to turn.
I also tried my Sightron SIII LRMDCM 10-50×60 scope the with Zero Point bore sighter. This Sightron did not move the crosshair when adjusting the focus / parallax setting or magnification setting.
My conclusion: With my two Nikko Stirling 10-5-x60 MKII scopes I can not use Chairgun to calculate elevation settings. I need to use real world observed elevation settings to click for the distances, and should not adjust the magnification setting. No big problem for target shooting at fixed distances. But not very handy when shooting Field Target.