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Thread: sight radius

  1. #1
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    sight radius

    I think alot of people don't know much about what sight radius is or why it matters. Here's a picture to help illustrate it.


    The sight radius is the distance between the front and rear sights. The picture above shows 3 16" ARs with 3 different sight radius. Carbine, mid, and disapator. As sight radius decreases, the error in sight alignment is magnified, resulting in a bigger miss.

    Again, no issue at all with a scoped rifle, but the sight radius alone makes a full length AR using open sights about 10% more accurate.

    If you look at the illustration below you can see how the miss due to the same sight misalignment grows as the sight radius gets smaller.


  2. #2
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    Well yes, that's true.

    BUT


    I would posit that the ultimate intrinsic iron sights accuraccy potential is not the sole driving force behind most AR sales, and there are valid reasons for going with the shorter handguards.

    What makes you think people don't understand?

  3. #3
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    I've just seen alot of posts where people expect their iron sighted M4 to be as accurate as an A2, or say that there is no advantage to a mid length AR and when sight radius is mentioned they have no idea what your talking about.

    I've seen people refer to relief distance for a scope and call that sight radius too.

    Competition shooters know how crucial it is, and specially built marksmenship guns often have a front sight that is extended beyond the muzzle in order to increase sight radius.

  4. #4
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    Well this is spliting hairs but what else are discussion boards for.

    I think an 16" Mforgery IS as accurate as an A2 (or as close as can be measured). The 4" barrel doesn't make that much difference. Now whether an individual shooter is more accurate with one gun or the other is a whole 'nother question.

    I also would venture to say that there are enough other factors in an AR that effect accuraccy that trying to pin down a particular rifles quirk to sight radius approches folly. Yes shorter sight radi are more sensitive to bad alignment then longer, but that's an easy fix. Align your sights.

    FWIW, I haven't seen any real difference in intrinsic accuraccy between A2's and real M4's on any .mil zero ranges. Individual rifles may or may not be accurate but as a type, the two are so close as to be hard to distinguish.

    Sight Radius is definatly a factor in being able to easily or quickly shoot accurately, but it doesn't make the rifle itself less accurate.

  5. #5
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    Just as important as the sight radius is the sight height. It controls the "sight roll" of the gun and while the alignment with the longer sight radius is good, if the height of the sight system is too much, the least "roll" of the firearm will cause a major offline aimpoint of the rifle. Realistically, the closer to the barrel top the sight, the better, and the longer the sight radius the better.

    You're pretty much limited by length of barrel for sight radius, but sight height can be changed by a number of things... whether or not the gun is a flattop, has a carry handle, or if the sight system is mounted for tactical usage. Some sighting systems are mounted on the side rails these days to accommodate different tactical uses, and while you might have a good "top sight" system, your "side mount flip ups" might actually be better because they're closer to the barrel... but you tend to lose sight radius with them because they're rail mounted.

    WT
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    I own both 20" and 16" barrelled AR's, and find no measurable difference in my accuracy. Granted, I'm limited to 200yd ranges in my area, but I doubt I'd find one significantly less accurate at even twice that range.

    Overstated.
    Last edited by kcshooter; August 19th, 2010 at 09:25 PM.
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  7. #7
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    I think it would be better to say a longer sight radius is easier to shoot well.
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    Re: Sight height. It seems to me height makes little difference. A bead on the barrel or a tall front sight (ala AR15) will both be in the same relative position with a 15 degree cant (roll) of the rifle. The only problem with canting (as long as it is the same each time) is the need to use both elevation and windage adjustments when adjusting up/down or right/left. Of course this can actually be a benefit if needing to adjust towards two o'clock (for right handed shooter0 as only the elevation screw may need adjustment. As with anything else, there are both positives and negatives.

  9. #9
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    Another thing to consider is the width of the front sight, relative to the eye. I know that on my Winchester levers, the front sight subtends (covers) six inches at 50 yards, or 12 inches at 100 yards.

    I like my levers to "shoot into" the front sight. That is, on a given target I put the dot on the target and fire the rifle. If I've done my part, the bullet will fall into that area covered by the sight bead.

    My target rifles are sighted differently and my Sharps has a globe sight, which brings a whole 'nuther set of considerations.

    But yeah, I agree that generally it's easier to shoot a longer sight radius.

  10. #10
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    Long sight radius makes for higher precision as does smaller apertures. Short sight radius and larger apertures make for quicker target acquisition and are appropriate for guns intended for shorter range use.

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    That is one big reason I was able to look like I knew what I was doing my first time at a Highpower match. Long M-1A sight radius and NM sights are a little more forgiving than the mousegun sight radius. Wish you could get 1/4 x 1/4 NM sights for the big gun like you can for the mouse.
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  12. #12
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    Potentially a thread jack, but I need some education and you guys sound like the ones to ask.

    Pat McCoy's comment: "The only problem with canting (as long as it is the same each time) is the need to use both elevation and windage adjustments when adjusting up/down or right/left."

    This got me thinking about my PSL rifle and accompanying PSO scope, which I'm trying to learn how to use.

    The manual I found online (http://www.dragunov.net/docs/pso_scope_zero_manual.pdf) states the following:

    "Windage correction:
    Because the scope is off-set to the rifle's bore, you may need to make a windage correction for distances beyond your 100 meter zero. You have two options to accomplish this task. Either turn the windage drum up or down or use the hash marks to the left and right of the center chevron in the reticle. You simply “hold over” left or right, aligning your target over a particular hash mark. Each hash mark represents one click on the windage turret which translates to 1.9 inches of movement at 100 meters."

    I guess I'm feeling rather dense right now, but I can't wrap my head around why the windage would change along with the range adjustment.

    Can anyone pound this concept into my gourd?

    Thanks.

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  13. #13
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    I have no experience with a PSL or PSO scope. What you're describing though seems to be because the scope is offset. The offset is causing your sight picture to go at a slightly different angle than the bullet trajectory. If you're sighted in at 100 meters, then that is the point where these two "lines" intersect. Anything beyond that and the windage will be slightly off because these "lines" aren't parallel with each other, as they would be with a traditionally mounted scope.

    Does that make sense? So, if you're sighted in at 100m, and your scope is offset 1" to the left, then I'd expect your rifle to shoot 1" to the right at 200m, 2" to the right at 300m, etc. That's my guess anyway

  14. #14
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    That makes sense, somewhat.

    It leads me to think that it would almost be simpler to not "converge" the windage line of the scope with that of the barrel.

    If the scope is a fixed 2" left of the barrel and no effort was made to converge the windage "axi" (is that a word, as in the plural of axis?) of the scope and barrel, then you'd have a "fixed" inaccuracy of hitting 2" to the right, but there would be no need for any windage adjustments to correlate with range/elevation changes.

    Is this an accurate postulation?
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  15. #15
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    I suppose in theory that yes, you'd then make the line of sight of the scope parallel with the path of the bullet, making you off by a consistent amount.

    PS, although it looks weird written down, the plural of axis is axes

  16. #16
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    If the scope is a fixed 2" left of the barrel and no effort was made to converge the windage "axi" (is that a word, as in the plural of axis?) of the scope and barrel, then you'd have a "fixed" inaccuracy of hitting 2" to the right, but there would be no need for any windage adjustments to correlate with range/elevation changes.

    Is this an accurate postulation?
    No, it is not, it is a trigonometric solution. As the range increases the angles must be modified to match.

    This is the same difficulty with laser sights,and why they are zeroed for a set distance and at other distances used for target marking.
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    I'm not sure I follow, swgunner. The above mentioned technique, if done perfectly, would result in a constant horizontal offset between the point of aim and point of impact. The sight picture and trajectory are moving parallel to one another, so the distance between the two should stay constant, no matter what the distance. Since we're only concerned with the horizontal portion of a bullet's flight, we're dealing with two straight lines. I realize that a bullet will never fly perfectly straight, but the above technique should yield results as consistent as a traditionally mounted scope, since those bullets don't fly perfectly straight either.

  18. #18
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    i learned about sight radius when i shot a long barreled super blackhawk right after a model 11.

    in days of olde it was thought culverins were more accurate simply because they were longer,but it was the sr that made them so.

  19. #19
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    then you'd have a "fixed" inaccuracy of hitting 2" to the right, but there would be no need for any windage adjustments to correlate with range/elevation changes.
    I think you mistake "holdoff" for inaccuracy. If you hit a consistent point, and change your point of aim to adjust for it, you're good. If you're spray painting the target and keep moving your POA, you're inaccurate.

    "Adjustable sights" are just that... and "fixed sights" are too. You can fix them both to some degree, but knowing your holdoff is important once you've pushed them as close as you can.

    I was shooting a Romak today, and it shot 10" high at 100 yards. The scope is set in meters, and the mil dot movement on the scope was about 3.4", so I changed my POA to adjust for the center of the target and it shot perfectly fine... 6 of 10 rounds into the 9 ring with a scope having no magnification... simply a rangefinder.

    Shooting holdoff is a way to move your groups around without touching your sights, and if you first shoot a "scalar" target divided into inches, you can see where your holdoff needs to be, and shoot accordingly. It's not rocket science by a long shot, but the differences in ammo can drive you crazy. If you roll your own, you can shoot consistently without a problem.

    WT
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    True patriots feel that there is no problem in our Republic that cannot be solved by election, windage and elevation, or superior firepower.

  20. #20
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    those that understand what the original post was about, well, understand.

    Those that don't............don't.
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  21. #21
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    After rereading the original post I think there is a problem between the question and illustration.

    It is true that equal misalignment will cause greater error at a similar distance with open sights, however the illustration shows a receiver which uses an aperture rear sight.

    As long as the aperture is smaller than the pupil of the shooter, the shooter's eye, not the rear sight mounted on the gun, is the true rear sight. Any misalignment of sights would be due to changing the cheek position on the comb. Apparent location of the front sight in the rear aperture has very little to do with where the bullet will impact.

    There is excellent information here: http://www.targettalk.org/viewtopic....lax+supression

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