Same shooter, same ammo, same gun, 2 different scopes, 2 different shot group size. What caused the groups to spread? If the only thing that has changed is the scope we have to look at what mechanically has gone wrong to make two distinct shot groups. Assuming that the scope and rings have been properly mounted, leveled and torqued to specs. What has changed? The scope. The reticle. The magnification. What affects how we aim the weapon system? The reticle? Now at this point I’m not talking about zero. I don’t care where my rounds are printing. All that I have noticed is that my shot group went from .75 moa to 5 moa. The Difference in reticles “could” be the issue. Each manufacturer has a different standard for how thick the stadia line are. Some reticles can be more than an MOA thick. How does this effect my shot group? Well if my POA is 1 MOA and my weapon system can shoot 2 MOA and with one shot I un-intentionally aim at the top of my MOA and next round I’m at the bottom of my MOA then my 2MOA gun just became 3MOA just by having a non-refined POA on the reticle. in the case of two different scopes if we only change reticles this could be the issue. What else could cause this issue?
The definition of parallax is “an apparent change in the position of cross hairs as viewed through a telescope, when the focusing is imperfect.” Here is an example to help you understand what parallax is. If I am sitting in the driver’s seat of a car and I look down at the speedometer and it reads true at 40 mph but if I jump over to the passenger seat and look at the same needle it will appear to be a different speed because of my angle of view, just like moving my head around on the rifle. Some scopes will be considered “parallax free.” What this means is that from the factory they have picked a range to set the parallax i.e. 150m. At 150 meters the image as a whole will be as crisp and as clear as possible and your target and reticle will be focused on the same plane. As you get closer or further the images will separate or not be as clear. If you do not look through the optic or have your head in the same exact position as a previous shot you could have a deviation from your previous shot.
Some scopes have shooter adjustable parallax. This allows you to adjust your parallax on the fly to help you precisely engage targets at various ranges. In order to ensure that your scopes parallax is set you must first focus your reticle. You will need to locate your diopter setting which will either be near the ocular lens or towards the front near the Objective lens. Then point your scope towards a clear back drop like the sky or a clean piece of paper. Then quickly glance through the optic then look away. The reticle should be instantly crisp and clear. If it is not, then make a bold adjustment on the diopter setting. Then look again. Continue this till you get a crisp clear reticle. When making, adjustments do not stare too long because your eye will force the reticle to be in focus. Now we need to check our parallax. Acquire your target and pick a finite point of aim on the target. Now move your head up and down like you are nodding your head yes. Make sure you do not move your weapon system. If parallax is not adjusted, you will notice your reticle will not stay on the precise point you are aiming. Use the parallax knob and move it in large turns and gradually work back and forth to smaller turns till the reticle stays were you are aiming. Now your parallax has been adjusted for that particular target. **make sure you adjust your parallax for every new target and anytime you touch elevation windage or focus knobs. In the case of two optics if we have encounter bad parallax adjustment, or failure to account for parallax, different reticles we could see this major shot group dispersion. ~ John Brady