Previously I had my ar shooting about an inch high at 50yds off a bench. I took it to 100yds and was shooting prone. It was shooting about 6 inches high at 100. I adjusted the elevation on my optic to bring it dead on at 100. Why did it get higher when going to a longer distance? Only thing I can think of is the 50 yrd shots were hitting the paper on the upside arc of a zero that was actually much farther. Could shooting position change it that much?
- Thread starter cpayton
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No, shooting position has nothing to do with it. A 50 yard zero will come back on the sight line a little past 200 yards and is a popular "improved battle zero". It is still rising when it passes the 50 yard mark. A rifle with a 50 yard zero will have the round hit several inches high on a 100 yard target. Since your zero was an inch high at 50, it will hit even higher at 100.
AR-15 Zeros and Trajectories
AR-15 Zeros and Trajectories
A lot is going to depend on relative heights of the target to the rifle barrel.
The bullet can't rise unless the barrel is pointed up to begin with. It is always falling from the time it leaves the barrel.
Shooting from the prone, at 50 yards your barrel is going to have a considerable upward angle to it. Of course this depends on the relative height of the target and the end of the barrel. Now, depending on the trajectory of the bullet at that barrel angle, your bullet may be rising still on its way to the target or not. 338 Lapua will have a very different trajectory than a 22 LR.
The bullet can't rise unless the barrel is pointed up to begin with. It is always falling from the time it leaves the barrel.
Shooting from the prone, at 50 yards your barrel is going to have a considerable upward angle to it. Of course this depends on the relative height of the target and the end of the barrel. Now, depending on the trajectory of the bullet at that barrel angle, your bullet may be rising still on its way to the target or not. 338 Lapua will have a very different trajectory than a 22 LR.
The iron sight and red dot zero on the M4s I qualified on before coming to the sandbox are 25/300m. The elevation dial on the rear sight gets set at 300m for the qualification. All shots for the zeroing are at 25m. A 2" optic height is a good WAG for a M4/AR-15 for a standard red dot. The 25m zero causes a steep barrel angle relative to the optic which is why it also is OK for 300m. The bullet is on a steep path upwards at the 25m mark. They are meant for minute of man accuracy and not precision work. I belive the same thing is happening with your situation.
Bullets rise coming out of the barrel and then drop, crossing line of sight TWICE no matter if barrel is level or inclined upwards. Run a ballistics program and prove it to your self. A typical 223 w/55gr bullet at 3000 fps rise, crosses line of sight at 50 yards an drops back to line of sight at ~200. It is about 1.3" high at 100. Center of scope for the proram is about 2 1/2" above barrel center line. A typical high power (say 270) scope is 1 1/2" above center line and will cross line of sight ~30 and 200. Straight ballistics.
There is always a small upwards incline to the barrel if you have it zeroed with any optic mounted above the barrel. If the barrel is perfectly level, the bullet will start falling as soon as it leaves the muzzel. In order to get a shot to zero with ANY scope mounted above the bore axis, the barrel will have to be inclined up. The incline will depend on the zero range, the height of the scope axis to the bore axis, and the external ballistics of your bullet. If the barrel is perfectly horizontal when the bullet leaves the muzzle, it will continue to drop because there is no force acting upon it to force it up. The only reason we can get a zero is the barrel is inclined up slightly and the force of the initial muzzle velocity combined with incline is more than the force of gravity. Then gravity eventually takes over at the apex of the ballistic curve and pulls the bullet back down. The laws of physics can't be broken.
Facts: The bullet will begin to fall as soon as it leaves the bore. There are three parts to external ballistics: line of bore, bullet path, and line of sight. When the bore is at a greater angle than horizontal then the bullet path, a constant and uniform arc, will intersect line of sight twice; but, the bullet path will never rise above line of bore. On an M4 zeroed for 300 meters the line of sight and bullet path will first intersect at 25 meters. This is how it is possible to sight-in the rifle at 25 meters for a 300 meter zero. Using a center of mass hold on the "E" target a shooter using the 300 meter zero could get a hit all the way out to 400 meters when allowing for the full 20 inches of target below center or mass hold.
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So we saying the bullet crosses line of sight Twice with a scope mounted normally and is why the OP can be zeroed at 50 and high 100. Probably would have to do unusual mounting of scope to have bullet exit barrel and always drop. Wonder if you would EVER SIGHT the scope in if it never crosses line of sight.
you are all missing the point here. The optics and the Barrel are not on the same plane. The optics are a couple inches above the plane therefore they are always angled to come to the same point. They are only accurate at that point, before or after will be off and have to be adjusted for. Whether the bullet rises or falls depends on how you look at it. You are assuming the optics are on plane with the barrel which they are not.
Here is a diagram that I got from Demigod's article (link below). I think you might be confusing the bore line and the optic line. If the optic plane is horizontal, the bullet can cross the optic plane once, twice or not at all; depending on the alignment of the bore plane to the optic plane.
PRACTICAL LONG-RANGE RIFLE SHOOTING - PART II: OPTICS
PRACTICAL LONG-RANGE RIFLE SHOOTING - PART II: OPTICS
The bullet doesn't "get lift'. The barrel is angled slightly upward, as depicted in this drawing from the link I posted earlier..
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The bullet doesn't "get lift'. The barrel is angled slightly upward, as depicted in this drawing from the link I posted earlier..
I was 1) being facetious and 2) hoping to bait WXL into a battle of the internets.
And your illustration is hardly germane to this discussion as it appears to depict an iron-sighted firearm rather than one equipped with an optic as described by the OP. (<--Again, being facetious.)
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OK, I see. The picture is a visual representation of what Sterling Shooter described. Maybe the picture will help those who believe the bottom of the diagram.
Pretend the eyeball is the optic and the irons are buis. Then it's germane.
Line of sight is line of sight whether irons or optic.
Why do you feel the need to make inaccurate comments and then say "just being facetious?". A rifle that is zeroed does not know if the sights are iron, RDS, or scoped, but it is a guarranty that the bore axis is on an angle above the line of sight, no matter what sight system is employed. That will not change.
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From Merriam-Webster online:
Facetious
1: joking or jesting often inappropriately : waggish <just being facetious>
2: meant to be humorous or funny : not serious <a facetious remark>
Or, I suppose your case, not.
I understand the English language very well.
Do you undertsand that "And your illustration is hardly germane to this discussion as it appears to depict an iron-sighted firearm rather than one equipped with an optic as described by the OP. (<--Again, being facetious.)" is an incorrect statement and a weak attempt at "facetious" humor, and that some others don't understand the concept that is being discussed?
Troll, dic*, or ignorant, I'm not sure, but you have provided nothing helpful.
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The old bullet rises discussion… you guys have fun. Some people just can't comprehend that whole science thing, even when you bring pictures.
One more time… Bullets don't rise, they are always falling.
One more time… Bullets don't rise, they are always falling.
They are essentially "lobbed" at a target beyond point blank range. They go up, and down, in an arc. That arc will intersect the line of sight on the way up, as well as on the way down.
Depends on the distance you have the weapon zero'ed at. Sometimes only intersects line of sight one time.
Other than a 100 yard/meter zero, which is close (but not exactly) to what you are describing, what zeroes don't intersect the line of sight twice? 25, 33, and 50 all do.
The 100 yard chart you posted shows the bullet touching the line all the way from 60 to 120 yards (actually far more than intersecting the line of sight twice), although it is a much flatter trajectory than the other zeroes.
It still rises and falls in an arc.
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Google some Bullet Drop Charts. I'm not screwing with you, but if you set point of aim/point of impact at the top of the trajectory, you have a zero with a lower impact on either side of the zero.
That is impossible.
No. Your ability to understand it apparently is though.
So you believe the round will cross the line of sight two times regardless of what your zero distance is?
Get to google-ing.
LtDan,
If the barrel is horizontal and line of sight is initially parallel to line of bore the bullet path will not cross line of sight. When the rifle has an attitude creating a trajectory to get the bullet to target distance and the sight is adjusted to appear as being on the target then the bullet path will intersect the line of sight twice. The only way you can get what you want is to have the rifle with sights not properly adjusted to create a trajectory to hit the target.
Bottom line is DWood is absolutely right; and therefore, you might as well be arguing the world is flat. Since you do not, it appears, completely understand, you might want to get some training in external ballistics and the concept of zeroing, in other words it is you who needs to do some Googling.
If the barrel is horizontal and line of sight is initially parallel to line of bore the bullet path will not cross line of sight. When the rifle has an attitude creating a trajectory to get the bullet to target distance and the sight is adjusted to appear as being on the target then the bullet path will intersect the line of sight twice. The only way you can get what you want is to have the rifle with sights not properly adjusted to create a trajectory to hit the target.
Bottom line is DWood is absolutely right; and therefore, you might as well be arguing the world is flat. Since you do not, it appears, completely understand, you might want to get some training in external ballistics and the concept of zeroing, in other words it is you who needs to do some Googling.
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Sterling,
hundred yard zero, POF P308 Hornady 168 AMAX TAP Nightforce NXS 3.5-15:
25 yards -2.5in
50 yards -1.5in
75 yards -.5 in
100 yards 0
125 yards -.25 in
Continues neg from there.
Trajectory and line of sight intersect at 100 yards. Bullet does not cross line of sight more than once.
AR (5.56) set up the same way, data is different. "sights" are set up properly and hit the target just fine.
Yes it does. It will intersect once before reaching max ord and once beyond max ord. Your example only needs that the ballistics table be magnified to understand the effect. There will indeed be two points, even in your example where the distance between intersections might be miniscule.
The big picture here is that a basic understanding of external ballistics helps to dispel myths which undermine good shooting.
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When there is an intersection of line of sight with a bullet path, which is constant and uniform arc, then the intersection of line of sight will be first in front of and again behind max ord. And in your example you would effectively have a point blank zero for what appears to be a flat trajectory over a short distance, but, you still have a first point of intersection and a second point of intersection for whatever the arc. Those two points are the actual zero distances. In other words, when actually zeroed for 100 yards the trajectory will place max ord at some place between intersection points for a 100 yard zero.
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With most trajectories the bullet's path will indeed cross the line of sight twice with standard zeroing distances. However it is possible to only have one intersection point. Just like an inverted parabola on a coordinate grid. The equation can cross the X axis once, twice or not at all. You can get any bullet trajectory to have only one intersection with the optic plane. You just have to change the zero distance to make it so.
From a constant and uniform arc, the line of sight will intersect twice. period.
I assumed you were being facetious so did not reply. Some of the posts indicated they really did not understand so hoped I could help. It is very helpful tool to understand the trajectory, having "zeroed" quite a few hunting rifles at camp or checked zero by firing at 30 yards (270, 300 WinMag, 300WSM) and 6.8 @ 40 yards. Close enough to kill deer at 200-250 yards.
I was also hesitant to reply to you as you were facetious or had to answer as if you were dumber than $h.. I chose to wait as I knew others would chime in. I figured I was still a newbie and probably get baited once in a while. I am 71 this week, love hunting, reloading and play with ballistics programs for all my loading. Load rifle from 22 Hornet, 223, 7-08, 7 RemMag, 270, 308, 300 WinMag, 300 WSM, 375H&H, 416 Rigby, 6.8 SPC, 284 Win, 44-40... may has missed some. Handgun from 380, 9mm, 357 mag, 357 Sig, 38 Spl, 44 Sp, 44 mag, 454 Casull, 41 mag, 10mm, 45Colt, 45acp, 40 S&W,...Yes, I must have something to shoot all of those.
So I am old enough to not take offense.
Note: Facetious comments may not help some who are trying to learn.
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Those who are insisting that the bullet path must intersect the line of sight twice, or not at all have either forgotten what they learned in geometry, or were never taught geometry at all. There is always a zero where the top of the arc of the bullet path rises to touch the plane of the line of sight without rising above it resulting in a single intersection. This can be proven mathematically.
If a rifle were to be zeroed as such, the only way the bullet would intersect the line of sight twice would be the result of an error in alignment or a deviation of the bullet from it's plotted path
If a rifle were to be zeroed as such, the only way the bullet would intersect the line of sight twice would be the result of an error in alignment or a deviation of the bullet from it's plotted path
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At what range would be that zero on a .308 rifle with an optic centered 2" above the line of bore?
+1
No offense to Sterling Shooter; but please explain your last comment of how an arc of ballistic trajectory must ALWAYS intersect the line of sight twice? Just as I related a ballistic curve to an inverted parabola equation on a coordinate grid; you CAN have only one intersection point on the line of sight. As MistWolf also explained, geometry plays a huge part here. Treat the X axis on a coordinate grid and the line of sight as the same. Also the inverted parabola equation and the ballistic curve the same. Both the optic plane and the X axis are horizontal. The ballistic curve follows a quadratic equation. The equation, or ballistic curve can intersect the X axis, or the line of sight once, twice, or not at all. It is basic geometry.
Here is a chart worked up by Sierra for the 168 gr MK at various velocities & zeros. The bullet path starts at -1.5" so LOS is 1.5" above the bore
http://www.exteriorballistics.com/ebexplained/4th-rifle_tables/pdf/308168hpbt.pdf
Menu of bullet charts
exterior ballistics
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It seems the OP's original question has been totally lost in the debate of the 100 yard zero, +/-, touching the line of sight once, or twice. The OP zeroed an inch high at 50 and wanted to know why his zero hits 6" high at 100. The answer is that his bullet is rising and falling in an arc that intersects the line of sight twice, once near 50 and the second time near 200. At 100, it hits high.
This is the case with every zero I know of other than 100 yards. My opinion, a 100 yard zero is the one zero that does not clearly break the line of sight, rise above it, and then reintersect the line of sight in a clearly measurable way. I also believe that it does not only touch the line of sight in a parabolic arc at only one point, exactly at 100 yards. IMO, it begins to reach maximum height slightly before 100, and continues along the line of sight until sightly past 100. In effect, touching the line of sight twice.
For those that insist it is only touching the line of sight once, OK. But if you are saying at 99 it is below and at 101 it is above, I disagree, and I don't think anyone can actually prove it either way.
Perfect for Internet debates.
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OK, you win.
So, which one is it?
Whatever you say it is.
OP, are you clear on why your inch high at 50 yard zero hits 6" high at 100?
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Travis Haley use to have a really good video about battlfied zero's and why they are preferred because of the best compat effective grouping. He went through and shot 5 different rubber dummies with different zeros and then from other ranges and he had very good descriptive detail in the video. I can't find it on youtube anymore but I believe it's from his adaptive carbine dvd. Anywho, if somebody can find it, it has excellent refferences as to the ballistics and different drops at different distances.
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