Does shooting position affect Zero?

[Enoch]

So I was reading through the stickies last night about setting Zero and I think I have a basic understanding but I need to re-read the rear sight elevation info a couple more times.

As I was thinking about all the great info in those stickies today and trying to process it all, I started thinking about shooting prone vs shooting at a bench rest. Since the 100 zero produces a flatter bullet path I'm sure this question is less relevant to that zero technique.

Let's say you decide to use the 50/200 zero method and let's assume that your 50 yard target is elevated 5 feet off the ground.

Since this method of zeroing produces more of a parabolic bullet path, it seems intuitive to me that zeroing at 50 yards from a seated bench rest would produce a different far zero than from a prone position. There would be maybe 3-4 feet difference in barrel height when comparing these 2 positions and thus the prone position would require a slightly increased angle above the horizon when compared to an identical POA from a seated bench.

Would this not produce a higher parabolic bullet path and thus a longer far zero? Or is it simply that there is a difference but it's not significant? Sorry if this has been answered before and I'm sure there's a simple answer but it has me a little perplexed.

[TehLlama]

Think of the point of aim as a line through three dimensional space, and the POI as basically where the parabolic trajectory of the bullet traveling through 3D space intersects with planes perpendicular to the POA line. If you're tracking on that, a lot of other stuff clears itself up very easily.

Shooting position relative to having a point where the point of aim through the sights or an optic coincides with the point of impact for a particular load through the rifle doesn't matter, as that point of aim of correct sight picture is just a line in space which is functionally independent of angle to the target when relatively flat (to within the accuracy cone of a projectile fired from a rifle barrel - up/down angle changes of under 1 degree on flat ranges are going to result on POI shifts of maybe an arcsecond or two).

The 100yd/100m zero doesn't produce a flatter bullet path, instead that zero simply puts your POA line intersecting with a POI range through a part of the bullet trajectory that is very flat (for 5.56 rounds out of a carbine barrel). The 50/200 (to 50/250 depending on muzzle velocity, projectile weight, and coefficient of drag) zero is just relying on an intersection point where a longer range zero (nominally a 200yd for a 14.5" barrel and M855 type external ballistics) also is useful at a closer range (50yd), and the point in between with the largest error where POI is above POA (about 112yd off the top of my head) is still within a couple of inches. This has very little to do with what absolute angle there is between gravitational force imparted on the bullet and the POA line used for the zero (still assuming it's going to be very close to perpendicular, since 5' at 150' would mean an angle of at most 2 degrees (arctan 0.033333), you're still well within the error cone of basically any 5.56 projectile at a usable muzzle velocity... and that's going from supported prone to a very tall person's standing. Extend that to 200yd (where you really want to do a 50/200yd zero from) and the difference is beyond trivial.

[ETA] The biggest difference is the movement in NPOA from different positions, which also really changes default eye position and that's going to be the biggest difference in zero between positions.

[markm]

For me, zeroing in a low prone position gives me a different POI when I shoot upright. I don't often shoot off a bench so I make the effort to build up my artificial support so that I'm zeroing in as high of a prone position as possible. (this is with irons specifically)

[DiveFlyFun]

Most common problem is the change in check weld when shooting from prone, standing, etc. So its important to get the weld locked in and use it for all positions.

[Failure2Stop]

Changing eye position behind iron sights when changing positions can effect the ability of the user to perfectly (read: consistency in relation to the eye position when zeroing) center the front sight in the rear aperture.

For tasks requiring a high degree of precision, changing the inertia behind the gun will change where the shot lands, even if the point of aim is identical. This is a commonly seen occurrence when the shooter goes from a rested bench position to an aggressive loaded-bipod prone position, or from an aggressive position to a less controlled expedient position. This relates to how the upper receiver moves in relation to the lower receiver during initial recoil when the bullet is still contained within the bore. Much of this difference is absorbed in the precision potential of a duty 5.56 with duty 5.56 ammo, but far more appreciable with a heavier recoiling cartridge in a platform that delivers higher precision. A bolt gun will generally show less effect in behind the gun inertia change since the firearm as a whole is generally more rigid. Much of this issue is minimized by application of consistent pressure on the front and rear of the gun regardless of position.

[ejb3]

I just learned from that response. Thank you for opining.

[Enoch]

Think of the point of aim as a line through three dimensional space, and the POI as basically where the parabolic trajectory of the bullet traveling through 3D space intersects with planes perpendicular to the POA line. If you're tracking on that, a lot of other stuff clears itself up very easily.

Shooting position relative to having a point where the point of aim through the sights or an optic coincides with the point of impact for a particular load through the rifle doesn't matter, as that point of aim of correct sight picture is just a line in space which is functionally independent of angle to the target when relatively flat (to within the accuracy cone of a projectile fired from a rifle barrel - up/down angle changes of under 1 degree on flat ranges are going to result on POI shifts of maybe an arcsecond or two).

The 100yd/100m zero doesn't produce a flatter bullet path, instead that zero simply puts your POA line intersecting with a POI range through a part of the bullet trajectory that is very flat (for 5.56 rounds out of a carbine barrel). The 50/200 (to 50/250 depending on muzzle velocity, projectile weight, and coefficient of drag) zero is just relying on an intersection point where a longer range zero (nominally a 200yd for a 14.5" barrel and M855 type external ballistics) also is useful at a closer range (50yd), and the point in between with the largest error where POI is above POA (about 112yd off the top of my head) is still within a couple of inches. This has very little to do with what absolute angle there is between gravitational force imparted on the bullet and the POA line used for the zero (still assuming it's going to be very close to perpendicular, since 5' at 150' would mean an angle of at most 2 degrees (arctan 0.033333), you're still well within the error cone of basically any 5.56 projectile at a usable muzzle velocity... and that's going from supported prone to a very tall person's standing. Extend that to 200yd (where you really want to do a 50/200yd zero from) and the difference is beyond trivial.

[ETA] The biggest difference is the movement in NPOA from different positions, which also really changes default eye position and that's going to be the biggest difference in zero between positions.

Thanks man, that helps a lot. You're visualization description in the first paragraph helps me make sense of it. Much appreciated

[markm]

For tasks requiring a high degree of precision, changing the inertia behind the gun will change where the shot lands, even if the point of aim is identical. This is a commonly seen occurrence when the shooter goes from a rested bench position to an aggressive loaded-bipod prone position, or from an aggressive position to a less controlled expedient position.

Good point. I've seen high speed film of just how much a rifle can move before the bullet leaves the barrel. Pretty amazing.

[Arctic1]

A zero is adjusting the sight line so that the trajectory of the bullet intersects it at specific distances.

For example, with a 50/200 zero you have depressed the sight line down, so that when you sight in on your target with the sight line being more or less horizontal, the bore axis is at the angle required for the projectile to intersect the sight line at 50 and 200.

The trajectory of a given load shot through a given weapon is the same, regardless of the zero distance.

The shooting position does not affect zero, but as F2S says, inconsistencies in eye placement and shooting positions can affect POI despite the same POA.

[TehLlama]

The shooting position does not affect zero, but as F2S says, inconsistencies in eye placement and shooting positions can affect POI despite the same POA.

Exactly. As much as I may have inadvertently made it sound like that's a shooter error type deal, it's basically unavoidable as part of the position changes, and knowing how those work tends to be the most helpful. For most shooters just getting a really solid zero in the position where they're most likely to extract enough accuracy potential to nice is the order of the day (prone, maybe off a magazine, for most fighting carbines is a pretty good answer for that). I'm weird, and sitting actually sets me up closer to a consistent zero for everything, and I know that in any kind of supported prone I wind up underneath the ideal eyebox location anyway and just compensate by using a higher spot on the reticle when I'm doing precision shooting (but this lets me get an even more consistent cheekweld out of the deal)... and that's just with the same relatively heavy rifle.

Lightweight barrels and overall light weapon systems seem to get affected the most , I think Mark nailed it with considering the high speed film of what's happening when you fire the thing - it's a violent action that causes a lot of really cool elastic resonant stuff to happen in various parts of the rifle, so I'd be pretty unsurprised that even with an exact matching POA at the parallax ideal range for the optic is still going to produce a slightly shifted POI depending on how much damping is placed behind the stock (or even in improvised positions if more can come in odd placed around the rifle).