There's been some discussion about how two different people shooting the same rifle resulting in the groups centering at different places. The differences in impact at 100 yards or so being an inch or more away from each other.
As this has been happening since shoulder arms have been available, the commonly believed reason is that people's eyesight is different and that difference causes them to get different sight pictures which means the barrel points to a different place for each person when the sights are at the same settings for each.
But this is another shooting myth. Here's why.
With iron sights aligned on the target to produce a given sight alignment and sight picture, the light rays comming from both sights and the target have a fixed relationship to each other. That same relationship is maintained as those light rays are focused by the eye lens onto the retina. They don't change at all. Even with corrective lenses (eyeglasses) that bend all those little rays of light to compensate for errors the eyeball has, the positional relationship of the rays of light from the target, front and rear sight are the same. There is nothing that discriminates between each that moves just one or two of the three and displaces them such that the rifle must be aimed differently.
Some folks may align different types of front/rear sight systems to different positions relative to each other. The most common system this happens in is the bead-front/notch-rear set of iron sights. Where the front bead appears in the rear notch, and where the bead is aligned on the target may well differ between people. The rifle may well zero at different places, but in this example, the difference is caused by each shooter using a different sight picture; sight picture being the visual image seen of the front and rear sights relative alignment with each other. When both front and rear sight are of the aperture type, the sight picture is much more uniform across several shooters; the front aperture appears centered in the rear aperture. For either iron sight system type and the same sight picture, there is only one axis extending from the front sight through the rear sight and continuing back over the buttstock. It is that axis that the shooter's eye must be in to attain the same sight picture for each shot. That axis only moves when the sights are adjusted.
If different folks use a different sight picture with iron sights, then naturally, the sights may need to be adjusted for each shooter to get a zero. If the same sight picture and alignment is used by all, the sight axis will remain unchanged.
When scope sights are used and they are focused on the target to be parallax free, both the target's image and the reticule are at the same optical plane. That combined image is focused by both the scope's eyepiece and the eye's lens onto the retina. It behaves just like the sight picture from iron sights except that there's only one image to align on the target; that of the reticule.
Some years ago, a group of us were discussing this subject and decided to conduct a test to see what the sighting errors were amongst several shooters. We anchored a barreled action on a bench, then positioned a bullseye target 25 yards away well aligned with the rifle sight's axis. The rear sight had 1/8th MOA clicks. Both post and aperture front sights were used. Some 20 people were each asked to look through the sights, then adjust the rear sight to attain a given sight picture; 10 times for each person. Rear sight settings were recorded, then averaged to determing what setting each person got their sight alignment and sight picture. After this test was completed, three very interesting things regarding data were noted:
So, if the center axis of the sight/target image remains the same for everybody with a given sight setting to zero a rifle at a given range for one shooter, why do different people end up shooting to different places on the target using the same sight picture/alignment when their aiming eye is exactly on that sight/target image axis? The difference has to be caused by something else that is unique to each shooter if group centers for each end up a different places on the target. What is different between each shooter? Their physical size, shape and the way they hold the rifle.
It doesn't take much of a difference in how a rifle is held to change its point of impact by 1 MOA. Benchresters shoot those PPC-size cartridges with light bullets in rifles that are just resting on sandbags. The only part of the shooter that touches the rifle is their thumb behind the trigger guard and forefinger on the trigger; squeezing that 2-ounce trigger ever so gently. When the rifle fires, it meets exactly the same resistance in exactly the same places and angles for each and every shot. Should one of these benchresters decide to grab the pistol grip in the conventional way, the groups will open up considerably; even more if the fore end is held by the other hand. A highpower shooter at long range keeps his front elbow in exactly the same place for all shots fired. And places the butt in the shoulder in exactly the same place for each shot. Just like the smallbore shooters, the way the rifle is held for each shot is as close as possible to the same tension at the same angle for each shot. Move that front elbow 1 inch and the next shot will be near 1 MOA off call; perhaps in the 9 ring. Even pistol shooters can change point of impact for a given sight setting by just changing their grip. When scopes are used, the sight alignment variables go to zero for everybody. But they still have to make sight adjustments to compensate for how each one holds the same rifle. To say nothing about how the trigger is pulled by different people, especially if it's in the 2 to 5 pound pull weight range, as far as how much zeros will differ amongst folks.
No two people hold and present resistance to recoil exactly the same for a given firearm. During the time the bullet is going down the barrel, the firearm is moving backwards. The direction and magnitude it moves before the bullet exits depends on how the shooter holds it. With the same sight settings, it is completely natural that two people will shoot the same firearm to different points of impact; they don't present the same amounts of resistance to the arm's recoil while the bullet is going down the barrel.
It isn't the way the sights look to the shooters that causes each to get a different zero with the same rifle. It is the way the firearm is held by each shooter that requires a different zero with the same firearm.