Height Over Bore

If I zero a 1/3rd optic to my 2.6" irons, and move my head up, the point of aim doesn't change. What do you say your height over bore is then?

What's even the point of absolute cowitness?

  1. 1 month ago
    Anonymous

    I think this is a good question. If you have an Eotech does the height over bore change if you look through the top of the optic vs. the bottom of the optic?

    Also, OP in a practical sense just go with lower 1/3.

    • 1 month ago
      Anonymous

      It's the same point of aim and eotech has zero parallax so it shouldn't matter. You can put the gun down and look through it however you want and it should still hit the same spot unless you touch it.

      Hell, if the window was somehow big enough, you could look all the way down and see it through the barrel.

      • 1 month ago
        Anonymous

        Right but then you've co-witnessed irons to it and the irons are lower, meaning they're aiming up more for that POI. This has to fuck up accuracy at different ranges, right?

        • 1 month ago
          Anonymous

          Cowitness irons are emergency backups, if you're running an optic the point is to use the optic unless it's broken. So compromises vs if they were primaries are acceptable. For rifles a lot of people do something else entirely, like flip up BUIS either flush or angled (if the former quick disconnect mount so optic can just be taken off in a few seconds, if latter can popup the irons whenever).

          I don't bother with cowitness on rifles are all myself because the backup role can be handled fine with something dedicated (or just doesn't matter). But it isn't weird to accept compromise in something you will hopefully never once use.

          • 1 month ago
            Anonymous

            This thread has convinced me for the first time ever that for flip up irons, absolute co-witness could be better than bottom 1/3rd.

            This is because there is no such thing as a parallax free red dot nor holographic. The implication being that if you sight in your bottom 1/3rd irons to your red dot, as many do, then you're actually sighting in a parallax error that will make your DOPE data wrong. It may be wrong only within a small margin of your shooting error anyway, making it moot but it's still wrong.

            You could correct this to an extent by sighting in your irons themselves without using co-witnessing to do it. That would be more accurate in a general sense anyway but you would still have to generate 2 DOPE graphs for a given round since you are asking the bullet to rise 2 separate amounts for your zero distance.

            Using an absolute co-witness would eliminate this. So I think it's best if you are using flip-ups. If you are not using flip-ups, meaning the sights will always be in your sight picture, then an absolute co-witness would just get in the way.

            Regardless, this thread has changed my mind and told me something that I did not know. I can't even recall the last time a thread on PrepHole did that. Thank you OP and thank you to the helpful anons.

            I have all mine setup as absolute co-witness and my AR’s, I was using my 11.5” with an FSB and fixed rear during a recent class when I had some issues with optic (No Loctite on QD Mount Torx screw) so I switched right away to irons.

    • 1 month ago
      Anonymous

      >does the height over bore change if you look through the top of the optic vs. the bottom of the optic
      yes.

      I'm not OP but talking academically though, what if you had a large red dot designed so the glass went all the way down to touch the barrel. Let's say the red dot is huge, 2" viewing window and the irons are integral to the barrel such that their bore over height is effectively zero.

      Because the center of the large red dot is an 1" over the bore but it still co-witnesses with the irons a 0" over bore. What does that do to their POI vs POA? It'll be the same at sight in for the optic but are they actually pointing up drastically more? Assume the red dot is sighted in to the distance equal to the top of the bullets arc, does it matter versus a sight in that has 2 zeros such as 50 yards and 20 yards?

      This is the kind of question that both makes me realize I'm not that smart and keeps me up at night.

      if the sight is truly parallax-free (most aren't despite the marketing) then the POI when aiming using the top of the sight should be 2" higher than the POI when aiming using the bottom of the sight. but this difference would be the same at all ranges. whereas something like a scope might have 0" difference at 100", an 8" difference at 200", an 16" difference at 300 yds...

  2. 1 month ago
    Anonymous

    It really doesn't matter much if your bullet impacts 3in lower on a man-sized target, especially when a quality sight and your accurate range estimation is compensating for 3in or more bullet drop.

    • 1 month ago
      Anonymous

      I'm not OP but talking academically though, what if you had a large red dot designed so the glass went all the way down to touch the barrel. Let's say the red dot is huge, 2" viewing window and the irons are integral to the barrel such that their bore over height is effectively zero.

      Because the center of the large red dot is an 1" over the bore but it still co-witnesses with the irons a 0" over bore. What does that do to their POI vs POA? It'll be the same at sight in for the optic but are they actually pointing up drastically more? Assume the red dot is sighted in to the distance equal to the top of the bullets arc, does it matter versus a sight in that has 2 zeros such as 50 yards and 20 yards?

      This is the kind of question that both makes me realize I'm not that smart and keeps me up at night.

      • 1 month ago
        Anonymous

        Forgot my mspaint drawing.

        Sorry, misread OP. Your height over bore if you moved your head up would now be 2.8”, because you are aligning the red dot and target at a different angle. But if you drop your head back down to iron sight height, then your HOB goes back down to 2.6”.

        The point of aim wouldn't shift if you assume no parallax. This means the arc of the bullet to poi is consistent and so height over bore effectively has not changed because you're still compensating for the arc of the bullet by the same amount.
        The question is, if you co-witness you're irons to the red dot, have effectively given them a higher bore over height? That seems ridiculous. So, you're actually aiming them up. If you're aiming them up then they'll be high by a consistent MOA? at the second zero? I don't know the math for that.

        • 1 month ago
          Anonymous

          Forgive my crude MS paint drawing, hopefully this helps visualize things anon

          https://i.imgur.com/J5dEdSZ.png

          Okay, I've gone back to mspaint.

          Obviously this drawing is exaggerated but you can see the question now.

          Green line = path of bullet
          Blue line = irons co-witnessed to red dot but from a lower bore over height
          grey line = sight in of red dot

          They all converge to the sight in distance the same but the paths diverge drastically after that. You can use a dope chart for your red dot but it won't be accurate for your co-witnessed irons? I think that's the idea that OP is trying to explore here?

          Damn, you beat me to it anon

          • 1 month ago
            Anonymous

            Yeah I feel incredibly stupid because my brain cannot deal with that.

          • 1 month ago
            Anonymous

            https://i.imgur.com/J5dEdSZ.png

            Okay, I've gone back to mspaint.

            Obviously this drawing is exaggerated but you can see the question now.

            Green line = path of bullet
            Blue line = irons co-witnessed to red dot but from a lower bore over height
            grey line = sight in of red dot

            They all converge to the sight in distance the same but the paths diverge drastically after that. You can use a dope chart for your red dot but it won't be accurate for your co-witnessed irons? I think that's the idea that OP is trying to explore here?

            But that's not how a red dot works. It's not a straight laser going back. It's reflected off glass so it moves with your eye.

            • 1 month ago
              Anonymous

              So I think the HoB should be where the red dot reflects off the glass.

            • 1 month ago
              Anonymous

              Yes. If you notice, in the bottom picture when you put your head down, the dot also moved down into the irons' "trajectory". So the height over bore becomes the same when you move your head down and the dot moves onto the front iron sight.

              When you move your head back up to center the dot, your height over bore increases again.

            • 1 month ago
              Anonymous

              So I think the HoB should be where the red dot reflects off the glass.

              Doesn't matter it's still pointed at the same POI at a certain distance. The question becomes significant regarding how POI changes with distance.

    • 1 month ago
      Anonymous

      >It really doesn't matter much if your bullet impacts 3in lower on a man-sized target
      unless you're trying to shoot him in the dick

  3. 1 month ago
    Anonymous

    I assume it would still be 2.6. If your eye naturally aligns at 1.4” height, then it’s better to also center your dot at 1.4” as you get less parallax error and you can see around the dot easier and move your head around more before the dot moves out of view. But if your eye more naturally aligns at 1.6” then you’re correct, there’s minimal disadvantage to using the higher mount.

    • 1 month ago
      Anonymous

      Sorry, misread OP. Your height over bore if you moved your head up would now be 2.8”, because you are aligning the red dot and target at a different angle. But if you drop your head back down to iron sight height, then your HOB goes back down to 2.6”.

  4. 1 month ago
    Anonymous

    HOB is drawn from centerline of the optic relative to your barrel. Stop making this dumber than it needs to be. Fucking autists I swear.

  5. 1 month ago
    Anonymous

    >the point of aim doesn't change
    the point of aim does change. for a parallax-free sight it changes by the amount you raised your head, but this amount is constant for all ranges, for a non-parallax-free-sight, the change in POA varies by range, getting worse at longer ranges.
    if the POA truly doesn't change then you don't have a parallax-free sight, you have one with parallax and you happened to be sighting at the convergence distance.

  6. 1 month ago
    Anonymous

    Okay, I've gone back to mspaint.

    Obviously this drawing is exaggerated but you can see the question now.

    Green line = path of bullet
    Blue line = irons co-witnessed to red dot but from a lower bore over height
    grey line = sight in of red dot

    They all converge to the sight in distance the same but the paths diverge drastically after that. You can use a dope chart for your red dot but it won't be accurate for your co-witnessed irons? I think that's the idea that OP is trying to explore here?

    • 1 month ago
      Anonymous

      >They all converge to the sight in distance
      they don't. check again more carefully. the eyeball is looking in exactly the same direction as the line you're drawing for the iron sights. your picture is exaggerating the scale, we're talking about typically an inch or less difference between the dot and the irons, which is difficult to see without magnification, and the difference is made more difficult because that's within wobble range or subconscious re-aiming range. aim the rifle with the irons, clamp it down tight, then check with the red dot, and you will see the dot is absolutely moving around on the target as you move your eye around behind it. it isn't much - again we're talking only an inch or two, whatever the size of the sight window - but it does move.

  7. 1 month ago
    Anonymous

    I really like the lower 1/3 especially on ARs. Recently got a 2.04 SKD riser and I like where it sits since the stock is also lower than a standard AR.

  8. 1 month ago
    Anonymous

    see
    https://www.breachbangclear.com/parallax-free-isnt/

    this whole thread is due to a misunderstanding of what "parallax free" really means. it doesn't mean the sight is always on target no matter where your eye is. it simply means the aiming error doesn't depend on the distance to the target. they're only "on target" if you are trying to hit something larger than the viewing area of the red dot. something like a person, rather than a bullseye.

    • 1 month ago
      Anonymous

      Yes. If you notice, in the bottom picture when you put your head down, the dot also moved down into the irons' "trajectory". So the height over bore becomes the same when you move your head down and the dot moves onto the front iron sight.

      When you move your head back up to center the dot, your height over bore increases again.

      So what you're saying is that yes, this picture

      https://i.imgur.com/J5dEdSZ.png

      Okay, I've gone back to mspaint.

      Obviously this drawing is exaggerated but you can see the question now.

      Green line = path of bullet
      Blue line = irons co-witnessed to red dot but from a lower bore over height
      grey line = sight in of red dot

      They all converge to the sight in distance the same but the paths diverge drastically after that. You can use a dope chart for your red dot but it won't be accurate for your co-witnessed irons? I think that's the idea that OP is trying to explore here?

      is accurate if you zero'd your red dot in with the reticle centered but you will in fact get a parallax error equal to that experienced by the co-witnessed irons if you shift the red dot down to them.

      That reconciles the perceived paradox and heals my brain. So, I'm just going to go with it.

      • 1 month ago
        Anonymous

        Well basically yes. In order for your dot to maintain the exact same zero distance as your irons whether you're centered or have your head down, there is naturally going to be a small degree of shift at any other range, I'm pretty sure.

        In practice, your dot will likely not be *exactly* parallax free even at your zeroed range, moving your head around will change the POI just a tiny bit at said distance. So your dot will actually have a slightly different zero if you move your head down to irons level. But with the better red dots, it's close enough that it doesn't matter.

        • 1 month ago
          Anonymous

          Well, at 100 yards and a reticle focal distance of much farther, I'm willing it call it zero shift.

        • 1 month ago
          Anonymous

          Well, at 100 yards and a reticle focal distance of much farther, I'm willing it call it zero shift.

          Willing to call it zero in practice but logically, yes it would have to move a few mm. So, yeah and thank you.

      • 1 month ago
        Anonymous

        no, I'm saying that if you erase the line from the eyeball and draw a new line from the eyeball parallel to the line along the sights (which is the line you would get if you're shooting cowitnessed) then you get what is actually happening. when you raise your eye the red dot rises to match, and it will rise on the target as well. but the error is small enough that you have to have the gun clamped down in a vice to see it, you can't see it just moving your eye around because the gun's wobble is probably greater than the effect you're trying to see.

        • 1 month ago
          Anonymous

          Nah dawg. This guy

          Well basically yes. In order for your dot to maintain the exact same zero distance as your irons whether you're centered or have your head down, there is naturally going to be a small degree of shift at any other range, I'm pretty sure.

          In practice, your dot will likely not be *exactly* parallax free even at your zeroed range, moving your head around will change the POI just a tiny bit at said distance. So your dot will actually have a slightly different zero if you move your head down to irons level. But with the better red dots, it's close enough that it doesn't matter.

          explained it better.

          • 1 month ago
            Anonymous

            No dawg that guy still thinks parallax free means no poi shift, but it doesn't.

  9. 1 month ago
    Anonymous

    This thread has convinced me for the first time ever that for flip up irons, absolute co-witness could be better than bottom 1/3rd.

    This is because there is no such thing as a parallax free red dot nor holographic. The implication being that if you sight in your bottom 1/3rd irons to your red dot, as many do, then you're actually sighting in a parallax error that will make your DOPE data wrong. It may be wrong only within a small margin of your shooting error anyway, making it moot but it's still wrong.

    You could correct this to an extent by sighting in your irons themselves without using co-witnessing to do it. That would be more accurate in a general sense anyway but you would still have to generate 2 DOPE graphs for a given round since you are asking the bullet to rise 2 separate amounts for your zero distance.

    Using an absolute co-witness would eliminate this. So I think it's best if you are using flip-ups. If you are not using flip-ups, meaning the sights will always be in your sight picture, then an absolute co-witness would just get in the way.

    Regardless, this thread has changed my mind and told me something that I did not know. I can't even recall the last time a thread on PrepHole did that. Thank you OP and thank you to the helpful anons.

    • 1 month ago
      Anonymous

      >but it's still wrong
      If you're worried about that sort of error then you're using the wrong type of sight. Red dots trade speed for "good enough" accuracy. The point of cowitness was not that you should shoot like that, but that it gave you a quick way to get your red dot kind of bore sighted when new, and gave you an easy way to verify that your red dot didn't get knocked off target after you landed on it when hitting the deck. It was quite literally "good enough for government work".

      • 1 month ago
        Anonymous

        Well I was agreeing with you to a point when I wrote.
        >It may be wrong only within a small margin of your shooting error anyway, making it moot
        However, I don't know anyone sees the value in back up irons being primarily as a double check for zero. Most people value them as an emergency option if your battery dies or you break your glass somehow.

        No dawg that guy still thinks parallax free means no poi shift, but it doesn't.

        Literally not what he said.

        Okay, since this is the thread for drop and ballistics type stuff. Are there any good resources for figuring out wind speed? Am I expected to carry one of those things that spin and a wind sock or what?

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