Same Firearm & Ammo, Different Zeros Across Several People

[Bart B.]

This is often mentioned and the popular reason seems to be we all "look through" or "see" the sight(s) differently.

Fact? Fiction?

 

[imashooter]

Fact. A firearm can be "zeroed" to perfection in a lab. When you enter a human into the equation, forget it. The way the weapon is held, etc influences point of impact.

 

[cdb1]

True.

 

[WestKentucky]

Not as much a question of visual reference, but very much a question of grip, flinch, wobble, way that they aim, etc.

 

[PapaG]

Why would this surprise anyone? No two people have the same physical characteristics, eyesight, nervous system. Results on target would be expected to differ.

 

[Bart B.]

Please explain the "way that they aim" and "eyesight" effects how a zero on a sight is attained.

 

[stringnut]

Open sights ,especially buckhorn or semi buckhorn, are open to interpretation of sight picture. Often referred to as taking a fine or course bead. The best open sights have the bead or blade so that it just has a bit of light around it and a clearly defined location for vertical placement. Aperture sights have much less margin for error if used properly. You simply look through a hole. However, there is still the question of where you put the front sight. I am comfortable with a center of mass hold. This is OK for hunting and works for me. It does not yield the best groups as you cover to much of the target. Most recommendations are to put the aiming point on the bottom of the target. This can vary significantly from person to person.

Scopes should eliminate these issues but they don't always. Parallax comes into effect. Have two or more people shoot a rifle at 25 yards that has a 200 yard parallax setting and you will see what I mean. POI can move inches from person to person. Generally with an adjustable objective scope set for the proper distance POI will vary little or none from person to person.

 

[Carl N. Brown]

I had cataract surgery in both eyes in 2006 and had to rezero all my iron sighted rifles.
So how you see the sights matters, and eyesight can vary.
(It was nice to look at the night sky and see stars again and only one moon.)

As far as people getting different points of impact, how you align the sights matters too.
I have a tendancy to zero square post front sights for an "apple on a fence post" sight picture (bulleyes being the apple, post even with top of notch).
With .22 rifles with bead front sights, I zero with top of the notch cutting the center of the bead, center of bead covering the center of target.
Target sits on post sight.
Bead sight covers target.
It's the way I was warped at an early age: posts are target sights, beads are hunting sights.

People will get different point of impact from the same sights, depending who zeroed the sights.

I have bought a lot of used guns and I swear I believe some people zero their sights to compensate for jerking the trigger or hunching their shoulder forward anticipating recoil. (I try to detect those bad habits and discipline myself not compensate by sight adjustment.)

 

[Bart B.]

Of course, aligning open or aperture sights different, flinching in anticipation of firing or not having scopes focused on target will cause errors. I should have clarified that. Note that scopes and aperture sights used correctly never have parallax issues. Parallax is a shooter problem, not a scope or aperture sight problem.

Let's assume these things don't exist and four top ranked competitors are shooting the same ammo in one rifle. With aperture sights and scope.

Why do their zero's have over a one MOA spread?

 

[WelshShooter]

Bart, of all the people asking this question I didn't think it would be you!

If you recall this thread, re-read your post # 39. Your post stated that the same rifle and ammunition would produce different muzzle velocities between shooters. Think of a ladder test for load development; the point of impact changes based on the velocity of the projectile for each loading. Since different shooters produce different muzzle velocities with the same ammunition and rifle, it wouldn't be a stretch to assume that there would be different points of impact between shooters, no?

 

[Bart B.]

Since different shooters produce different muzzle velocities with the same ammunition and rifle, it wouldn't be a stretch to assume that there would be different points of impact between shooters, no?

What if muzzle velocity was the same for all but still required different zeros for each person more in windage than elevation? A 1% to 2% spread in muzzle velocity won't cause a 1.5 MOA change in windage.

 

[BrocLuno]

Put the rifle in a sled. Let the same people sight it. The spread will be smaller.

It's physiology, muscle density, hold and recoil absorption, etc. It's like asking two bowls of jello to stay the same when you shake them

 

[Bart B.]

Led sleds and other machine rests are great for load development. Because barrels whip different in such devices than when humans hold the rifle while bullets go through them, they cut no mustard for getting zero's on sights.

 

[X-Rap]

It is more pronounced in comparing right and left hand shooters so I suspect it is due to hold and the fraction of a second the bullet is in the barrel while in recoil.
Groups are similar but point of impact is not.

 

[herrwalther]

True. I have watched soldiers shoot a marksmanship test and seen rifles traded or otherwise borrowed that results in different scores. When I worked on a gun team for the 240B, I was an assistant gunner so I carried ammo for the 240 but still had to qualify on both the M4 and 240. I zeroed my M4 and let my 240 gunner borrow it to do his M4 qual. By coincidence I was able to do my qualification on the very same interval when a spot opened up. Rather than finding my gunner and swapping out rifles, I grabbed my team leaders rifle to score with. I normally shoot 38 or 39 out of 40. With my TLs rifle I shot 31/40. I retested on my issued rifle a few hours later to get 39/40. My AG shot 25/40 with my rifle and 30/40 with a different rifle he borrowed later.

 

[horsey300]

What if muzzle velocity was the same for all but still required different zeros for each person more in windage than elevation? A 1% to 2% spread in muzzle velocity won't cause a 1.5 MOA change in windage.

Funny, that's EXACTLY what happens when the .243 swaps between the bosslady and I, to my "zero" she will repeatedly put neat groups in perfect vertical alignment, but exactly 1.5 moa to the right. Doesn't matter if it's 100 or 500 yds, ALWAYS 1.5 MOA TO THE RIGHT. Why? I can't prove anything so maybe you could shed a little light on it? I figured her longer neck was the physiological reason.

 

[Bart B.]

The more the center of mass there is to one side of the rifle, the less the bore axis will swing before the bullet leaves.

If the center of mass is in line horizontally, the barrel will go straight back before bullet exits.

Mass equals rifle and shooter. Your wife weighs less than you do. She twists easier.

I use the same windage zero's standing and with a sling from sitting and prone. Same 308 Win ammo and rifle resting atop bags and shouldered at a bench prints 1+ MOA right at a hundred. My 30 caliber magnums' printed 1.5+ MOA right.

Same physics that makes a firework pinwheel spin. It's jet engine at right angles to its radius makes it spin one direction. Move the engine straight across to the other side and it spins the opposite direction. Put the engine at the center and the wheel would not spin; jet axis no longer acts like a force on a lever arm like a crowbar has.

Here's a program to see what happens with center of mass below bore (recoil) axis. It's right angles to horizontal. Use a 1.2 millisecond barrel time for the 308 cartridge used.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

 

[1911 guy]

Cheek weld. The shape of your body affects it. Two people standing side by side are different height, build, reach, etc. This is why sight adjustment changes when you change position, such as off hand to prone or kneeling. Eye sight, etc., all figure into it, but i suspect cheek weld and the differences in body type that cause it are the main culprit.

 

[entropy]

My Supply Sgt. and I shared the same M16A1, sighted in for me. I shot 38/40 with it, she shot 30/40. We were in a Med Bn., and TOE had one rifle for every two enlisted. Course, I had a trunk full of S&W revolvers nobody wanted, so there was that.... I had a 60 on tap from another unit in the BN if needed.

 

[mjsdwash]

I can tell you from people who insist on tweaking my sights so they can fire 12 rounds that it happens. Wish they would ask first, or not do it at all

 

[entropy]

I can tell you from people who insist on tweaking my sights so they can fire 12 rounds that it happens. Wish they would ask first, or not do it at all

My son kept doing that to my .223's scope....