The eyes don't have it...

[Flexmoney]

Source: http://www.recguns.com/Sources/XIIC2.html

XII. Accuracy

C. Miscellaneous

2. Two Shooters, Same Rifle, Different Point of Impact

by Bart Bobbit (hartb@hpfcla.fc.hp.com)

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:

When a post front sight was used, each shooter's sight settings had about a 1/2 MOA spread in windage and about a 3/8 MOA spread in elevation.

When an aperture front sight was used, each shooter's sight settings had about a 1/4 MOA spread in both windage and elevation.

With both types of front sights, the average rear sight settings for all 20 people were within 1/8 MOA of each other.

Some of these folks wore corrective glasses with rather strong prescriptions, others had mild prescriptions or didn't wear glasses at all. All of which indicates that once the sights are set on a rifle, the angular separation of bore and sight axis remains the same for everybody. It doesn't make any difference in how folks eyes vary. If there was a difference in apparent sight picture/alignment across these test subjects, it would have shown up in the recorded rear sight settings. OK, there was a 1/8 MOA difference, but considering the human eye resolves down to about 1/4 MOA, I consider that difference in the noise level.

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.

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[Jadeslade]

Really interesting article. I am death gripping.

[lugnut]

VERY good article. I've had many discussions with shooter over this and have often wondered why groups varied yet couldn't understand how my sight picture could give someone else a different POI!

[Flexmoney]

Yeah, I know I have often heard people telling me how their eyes "see differently"...to explain away the reason their sights are hanging half way out of the side of their gun, or some such thing.

The experiment above proves that to be nonsense. They took a fairly decent number of shooters, with multiple samples...shooters that would exhibit different points of impact...shooters with different degrees of vision and correction...and they limited the test to one main variable, sight on target.

I have often preached on what is left over...the shooter's interaction with the gun.

[Bill Nesbitt]

If the weather ever warms up I'll try this again. But I remember that when I was learning to shoot with both eyes open instead of one, that my groups would move a couple of inches depending on how many eyes I had open. That's shooting slow fire groups with a handgun at 25 yards or so on an IPSC or IDPA target.

[Flexmoney]

Bill, that would have to be another variable being introduced.

It sounds like an eye dominance thing. (Also, test looking through the edges of your eye glass lenses, vs. looking through the center. You gotta keep the gun in the same place though.)

What if we think of a laser or dowel rod...coming out of the target's bullseye... proceeding to just touch the very top of the front sight...go through the notch on the rear sight...and it continues on...all in a perfectly straight line. The eye that is doing the seeing has to be on that line.

The target, front sight and rear sight have to line up...whether we are there or not.

[el pres]

Great article, many times I have thought about this and expirienced it playing around on the bench.

I always thought it was just a paralax issue in where your eyeball is like with a shotgun, just your checkweld being

compromised. I'll have to play around with arm positions next time I go bench my rifle ?

[Ron Ankeny]

When scopes are used, the sight alignment variables go to zero for everybody.

Now I am confused. I had this discussion with an optometrist and with an engineer at Zeiss. Both of them told me the point of impact can shift for the same shooter holding the rifle exactly the same way if they switch from a flat to a curved corrective lens with different prisms. Who do I believe?

[Bill Nesbitt]

If everybody's vision is the same, then why am I wearing glasses?

[Flexmoney]

When scopes are used, the sight alignment variables go to zero for everybody.

Now I am confused. I had this discussion with an optometrist and with an engineer at Zeiss. Both of them told me the point of impact can shift for the same shooter holding the rifle exactly the same way if they switch from a flat to a curved corrective lens with different prisms. Who do I believe?

The context of the part you quoted was scoped vs. irons...there is no front to rear sight alignment to factor in with a scope.

[Flexmoney]

If everybody's vision is the same, then why am I wearing glasses?

The article does not say that everyone vision is the same? It point out that a variety of vision was tested. That is how it read for me...but maybe my eyes are different...lol

[Ron Ankeny]

The context of the part you quoted was scoped vs. irons...there is no front to rear sight alignment to factor in with a scope.

Yeah, I know. I am talking about scopes. I suppose the confusion comes from the other thread about irons on handguns.

FWIW, I experience a point of impact change when I switch from my tinted shooting glasses to my clear everyday glasses. The eye care and optic "experts" tell me the shift is due to parallax that is introduced because of the difference in correction for double vision and the curvature of the lens. The article you posted seems to indicate if the reticle is at the same point of aim and the way the shooter mounts the gun is constant, the point of impact will be the same regardless... I wonder, if a shooter's eye doesn't change in relation to the scope will the point of impact change if he/she messes around with the focus on the eye piece and the side focus parallax adjustment? If so, couldn't the point of impact change if different lenses are introduced between the optic and the shooter's eye? Hell, I don't know.

[Flexmoney]

I think the point of the article is that light travels in a straight line...like a laser beam. (Unless something distorts it, which convex and concave lenses could do.)

But, the big variable that was eliminated in the test...with shooters of all eye types...was the interaction with the gun. Once that was eliminated...coke bottle glass wearers and eagle-eyes all adjusted the sights the same.

[Youngeyes]

If you introduce an ophthalmic lens ( eyeglasses) between your eye and the scope, and you are not looking through the optical center of the eyeglass lens, you are getting a prizmatic effect which will move your view of the target. Have your Dr. mark that optical center on your glasses for you and you should see a differance. The best way is to use an RX lens set on the scope, if the scope can be made with an RX ring add-on. If you can set the scope to correct your vision with the diopter lens on the scope, use it without your glasses. Couldn't hurt. And you thought you were confused before. Always room for more confusion.

[Ron Ankeny]

Always room for more confusion.

I think I'll just have to shut up and shoot.

[Downrange58]

I think the point of the article is that light travels in a straight line...like a laser beam. (Unless something distorts it, which convex and concave lenses could do.)

But, the big variable that was eliminated in the test...with shooters of all eye types...was the interaction with the gun. Once that was eliminated...coke bottle glass wearers and eagle-eyes all adjusted the sights the same.

I do believe I am a firm believer of the relationship between the shooter and his/her interaction with the gun. I was shooting a tight group off of the bench the other day and with the same exact sight picture when I pressed the trigger, I swear I felt the one that got away high and right. I felt as if my grip pressure had changed and as the bullet slid down the barrel, I felt the gun move in my hand as what felt like torque changed the point of impact from what I was sure was the point of aim. It was great to experience it as I had read about it and now I had actually felt it. Cool stuff!