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.
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.
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)
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.
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.
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.
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).
The generally faster lock time of the bolt action (Rem 700 type trigger) helps the bolt to be much more forgiving of changes in shooting position in relation to zero. .308 gas guns are not the easiest of platforms to extract precision and form needs to be pretty much spot on to get the real potential.
In “practical” applications with an AR, I think body geometry/position/technique definitely plays. Doing the 50/200 zero from a bench, then going into a training class where they checked zero from prone/seated, it was obvious to me that there were differences since I had to fine-tune zero a bit. YMMV.
If all you do is shoot from a bench all the time, it probably doesn’t matter that much… but once you start to shoot offhand and such, little things add up in a hurry, imho.
This is pretty easy to test at most ranges though… shoot from the bench and zero, then prone out and check… that’s what I do from now on for AR purposes sighting-in a new setup.
Zero has to do with aligning the optic with the trajectory. You changing your position from seated using a bench, to prone unsupported has no bearing on this. It can/will affect how the shooter interfaced with the gun, but it does not alter the zero.
I beg to differ. If you do anything with a firearm that changes how the recoil affects the barrel position before the bullet leaves the barrel, you will definitely affect the point of impact relative to the sight picture. For example, if you held the rifle down on a rest for one shot and made another shot holding the rifle offhand, you can expect different points of impact even though the sight settings have not changed. To me, a change in point of impact with no change in sight settings is a change in the “zero”.
One needs to adjust the sights on a firearm using the same shooting style as you intend to use when shooting “for real”. I always advise folks who are “sighting in” their hunting rifle to hold the rifle the same way they would be in the field when adjusting their sights.
Yes, POI change will occur if you change the way the shooter interfaced with the gun.
Regardless, the POI shift you are describing will not be significant. And it is not a change in zero.
Say you have two guns, A and B. A is zeroed at 200, B is zeroed at 300. They have the same optic, and shoot the same load.
How much of a difference is there in POI when you shoot gun A at a 300 meter target, compared to gun B at a 300 meter target, when holding dead center?
Recoil affecting the barrel? I am not sure I see how this relates to the zero trajectory of a weapon. Barrel movement while the projectile is still traveling down the bore will affect the group spread, greater so if the shooter has a crap shooting position. Again, nothing to with the zero of a weapon.
A zero means that you adjust the sight line, so that the bore axis is at a sufficient angle for the bullet to intersect the sight line at a given distance.
Take guns A and B again. The actual trajectory of the bullet required to hit a target at any given distance is the same for both guns, regardless of zero. However, when shooting them using the zeroed sights, POI will be different at different distances because of how the sight line, and thus the bore axis, is different.
In my experience your zero will be different when shooting off the bench, than shooting from the prone position. It will also be different if you rest the rifle on a sandbag, then shoot from a standard prone position with a sling. Change in sling tension will also have some effect on zero, but most people can’t tell the difference.
You will also see a zero shift if you do not have consistent cheek weld. You mentioned “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.” My first question would be “is the weapon 5 feet off the ground?” once you get into your shooting position. Shooting at down or up angles has an effect on POI.
Changes in lighting conditions also effect POI. The change in lighting on both the sights and target have a marked effect.
The way the weapon reacts during recoil, due to your shooting position, will also have a noticeable effect on POI. Try it yourself. Move the buttstock from the inboard position to a place on the outside of your shoulder or on your upper bicep and note the difference in POI. Anyone who has done some High Power shooting will tell you that they sometimes have to make windage adjustments when moving from the standing to sitting or prone position, even on a reduced 200 yard course of fire.
Like tehlama pointed out, the projectile travels at an arc from the time it leaves the muzzle. The projectile will intersect with the sight plane as it travels down range. Your job is to make sure the arc intersects with the sight plane at the moment when the bullet passes through the target.
If you are just banging away at 100 yards and closer, most people would not be able to tell the difference. Once you start shooting sub MOA groups at targets out to 300 yards and beyond from different shooting positions, you can tell the difference in POI.
Goes beyond that - floating the handguard just restricts that cantilevered mass to only one point (that has a considerable amount of material at the barrel nut-upper receiver mating point) to affect harmonic oscillations of the rifle - it’s a LOT better than having two points, but just free floating a barrel doesn’t disconnect either piece from that vibration equation as the force of an explosion gets channeled out a chiral opening in a tube with a movable copper jacketed lead seal - stuff is still going to deflect as that much force is getting channeled places, no matter where or how it’s under tension from different stuff, just changes exactly which direction and how much it’s going to move.
That sling has to mount to something, even if it’s a FF handguard it’s going to affect POI because that force is still actually being imparted onto the barrel, just at a different spot, at a different angle for the force vector, which in this case just makes for a much more consistent (and therefore predictable) POI shift.
Unless there is a definition issue going on in this discussion, a zero cannot change unless something changes with the optic/sights, or bore axis alignment. With regards to the latter, shooter induced POI shift from barrel influence is not relevant when discussing zero.
When changing from a benched, seated position to a prone position, neither of the above are influenced.
I agree that POI shift can come from a variety of issues, but I believe it is important to not mix terms.
