So, you’ve zeroed your scope but your POI is moving when you zoom in, Mike Morton explains what you need to do…
Q. I zeroed my scope on mid-power (6x) at 30 metres, but when I checked it at the same distance with the mag turned up to 10x, my point of impact had moved low and left. Why does the POI move when I zoom in?
A. In theory, the point of impact at your set zero should remain the same whenever you change magnification. Remember, we are not talking about holdover changes, just dead-on zero. However, mass-manufacturing tolerances and the complexity of the moving elements within the lens arrangements of the scope means the accuracy of machining shows up when you alter anything. Expensive scopes cost more because of the finer tolerances within the manufacturing process.
Before you can begin to achieve good results at varying ranges and magnifications, you must ensure your scope is aligned with the barrel correctly, and that you hold the rifle correctly. Any slight misalignment will show up as you increase the range. If your crosshair is absolutely perpendicular with the bore, then at range you should see no change in impact other than drop.
You need to minimise the amount of deviation you get from this moving of the point of impact. If your scope has inaccuracies built in, you cannot take them out, you can only minimise their effect. One way to do this is to zero at the highest magnification: that way, as you reduce the magnification strength, the crosshair covers more of the target and the movement of the point of impact is less noticeable. This phenomenon is one of the reasons I zero my hunting rifles at one magnification – 6x – and leave well alone. Fiddling with anything only increases the chances of disturbing something in the equilibrium.
To test your scope for adjustment accuracy, you can do what is called ‘shooting the box’. Set up a target at your zero range. Fire a three-shot group dead-on. Adjust your scope 20 clicks up and 20 clicks right and fire another three-shot group. You should have another cluster in the top right of your target, about an inch or so from centre. Now do 40 clicks down and repeat the group; then 40 clicks left, followed by 40 clicks up. Finally, adjust 20 clicks right and 20 clicks down and fire another group. You should have your group absolutely back on zero. If there is any shift of zero during this routine, you know your scope is not adjusting perfectly.
As scopes get used over time, these adjustments can get slack as the springs holding the erector mechanism wear, along with all the other mechanical moving parts. The better the quality of the scope, the better the adjustments work and the longer they remain accurate. That, along with optical quality, is why the old adage “pay twice as much for the scope as the gun” came about.