Airgun Accuracy

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Chairgun and “Scope Height”

I hear a lot of people complaining about the ballistic software Chairgun not being in line with their real world results. But in my field target rifle and scope combo’s it has improved to being very accurate by measuring “scope height” in the right way. When I gave other field target shooters the advise to do the same thing they also find that Chairgun is very accurate. And that the input value for “scopeheight” was the parameter for which they had used the wrong input.

“Scopeheight” is a major affecting parameter in every ballistic calculation. Much more so than a few hundredths of BC or ten FPS.

“Scope height” is actually the wrong name for this parameter. The actual parameter needed is the difference between the line of sight and the center of the barrel at the MUZZLE. See the picture for the explanation of the difference.

Wrong way and Right way to measure scope height for Chairgun

Wrong way and Right way to measure scope height for Chairgun (picture courtesy of Brian Samson)

The right way to measure this “scope height” is:

  1. Mark a cross on something like a Yellow pages or some other sheet of paper.
  2. Set your scope to the minimum parallax adjustment distance and minimum magnification.
  3. Cover the Objective lens with tinfoil and poke a very small hole (0,12″ or 3mm) in the center of the foil. (this will greatly improve the sharpness of the image at close distance and will allow you to see the cross on the paper at close distance)
  4. Sight the cross on the yellow pages and with your muzzle as close as possible to the yellow pages fire a pellet.( I managed to get as close as one feet / 30 centimeters between muzzle and paper)
  5. You can now measure from the center of your pellet hole to the cross you’ve marked on the paper.

This will give you your scope height for Chairgun. (Originally posted by Harry’s Lad, the programmer of Chairgun).

small hole in the foil

small hole in the foil

The difference between Point of Aim and Point Of Impact (POI) gives the “Scope Height”

For me this improved Chairgun to the point that it is more accurate than I can shoot my groups. I use Chairgun for Field Target shooting from 8 out to 55 yards to make my turret tapes.
Of course you have to make sure your scope’s click values and mildot spacing are correct as well. My Big Nikko for instance did not have the 1/8 moa clicks that the turrets said. I measured them and they are 1/7th moa clicks actually.

Regards, Sven.

(Pictures by Steve in NC from the Yellow Forum and Brian Samson from Shooting-The-Breeze original thread click here)

 

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30 thoughts on “Chairgun and “Scope Height”

  1. Hello,
    I tested this method to setup my Steyr LG110 FT and it works absolutely fine. I tested clicks of practice for each meter of 8 to 50 meters and they corresponded exactly to the table shown in the program.

    Thanks for sharing
    Vasil

  2. I put out targets at 7, 8, 9 and 10m and measure how many millimeters the shot hits below the aimpoint. I then adjust the Chairgun scopeheight parameter so that the Chairgun curve corresponds to the actual measurements.
    My experience is that at short distance this experimentally determined scopeheight (and at long distances the pellet BC value) determine whether the Chairgun curve matches the pellet trajectory. Velocity and BC are unimportant for the close distance curve, whereas scopeheight, within normal height range, is irrelevant for the long distance curve.

    • Hi Jo,
      Thanks for your comment. I agree that your way of calibrating Chairgun at 7,8,9 and 10 meters will work as well. Although I have not tested this myself. It is more elaborate because you need to shoot at four different distances on four different targets. And you need to fiddle around in Chairgun. But it may be a good option when the scope does not give a clear enough image at 1 meter distance to see the cross on the paper.
      Sven.

  3. Hi Sven,
    i guess 2 distances is enough for the way i do it.

    Another appraoch is ordinary trigonometry. Example: suppose you can actually measure accurately the axis of the scope (sight line) with the axis of the barrel. (Even with a breakbarrel and some barrel droop these assumptions hold.) And assume the drop of the pellet to the first zero point is neglegible. The distance from the scope-objective to the muzzle is relatively small compared to the distance from de muzzle to the first zero point.
    Example:
    Distance scope-axis to barrel-axis at the scope: 3.9 cm
    Distance scope objective to muzzle: 50 cm
    Distance scope objective to first zero point: 1500 cm
    Then the corrected scope height at the muzzle would be: 3.9 x (1450/1500) = 3.77 cm (so just over 1 mm difference)

    Other, more extreme example:
    Distance scope-axis to barrel-axis at the scope: 5 cm
    Distance scope objective to muzzle: 60 cm
    Distance scope objective to first zero point: 1300 cm
    Then the corrected scope height at the muzzle would be: 5 x (1240/1300) = 4.77 cm (so just over 2 mm difference)

    So for most shooters, accurately measuring the distance scope-axis to barrel-axis and substraction of 2 millimeter would be a reasonable approximation. The challenge of course being the accurate mesurement. The way you and i do it is a way of avoiding this scope-axis to barrel-axis measurement, which can be difficult in some setups. 🙂

    Jo

  4. I take the sum of half the diameters of the scope tube and probe and add the distance between the top of the probe and the bottom of the scope tube. Came to 1.55 inches. Then did the point and aim method and the distance between poa and poi came to 1.55 inches. Coincidence?

    • Hi Tony, Thanks for sharing your observations.
      I have done the same thing twice. For the Steyr with a Nikko and the Walther with a Leupold. And in both cases the difference was significant. 1.2 and 1.4 cm or 0.47″and 0.55″ difference between the measuring diameters method and the shooting method described above. Those differences cause a difference in pellet drop at 50 meters/55 yards in Chairgun of exactly the same distances of 1.2 and 1.4 cm.

      So I would say it is coincidence in you case. And when I look at the pictures above it seems to me it actually impossible for the two distances to be the same (between scopetube and barrel and between muzzle and line of sight). Or your scope must adjusted so that the sightline is absolutely parallel to the barrel. But in that case you would not hit anything at any distance when using the center of the reticle.

  5. i am having an issue with my chairgun calculations. it may be related to this but its a drastic difference. rifle is zeroed at 35 yards. scope centerline is 2″ above bore. chairgun calculated approx. 2 mil hold over at 70 yards. in actually shooting the pellets land right at 1 mil under crosshairs. so I recheck all my figures. they are accurate. BUT…. I am using the fx no limits scope mounts….. and I have angle built into the scope at the moment for long range shooting. so the scope is angled downward in relationship to the bore. any input on how this will change my figures and how I can correct either the gun or the software to compensate for this. I know its not uncommon of shooters to do. ted from teds hold over uses them extensively. so now I need to figure out how to fix this problem.

    • Hi Scott,
      Thank you for your question and input. To be able to answer I would like to know 2 things:
      1. Is your scope at the correct magnification for the mil’s in the reticle? If your scope is a 2nd focal plane reticle scope it will need to be at the right zoom / magnificaton setting to read the correct mil distance from the reticle.
      2. Did you determine the 2 inch scope height in the way described on this page? If you did it in a different way (by measuring the distance between scope tube and barrel) the adjustable mounts will increase the difference between the measured scope height and the value that Chairgun needs (distance between line of sight and the centerline of the bore at the muzzle). The downward angle created by the adjustable mounts will reduce the distance between line of sight and the centerline of the bore. Even more when the elevation turret on the scope is set to a long range setting. I use the Sportsmatch ATP61 adjustable scope mounts myself. And using the the method described here gives me a perfect match between Charigun and real world shooting. (and I did a calibration on the click value of my scope, which proved to be 1/7th moa instead of the claimed 1/8th moa).

  6. 1 – Yes my scope is at its calibrated “mil” correct setting. It’s a second focal scope but it’s true to mil at 14 power. I also verified this with a 3.6″ target at 100 yards. The target is exactly 1 mil in height.
    2 – no I did not perform this system …. Yet. However in trying various settings in chairgun I am still not able to make the scope height match my shooting results.
    In brainstorming I feel I may not have my “far” zero. I possibly am experiencing my “near” zero and its throwing the calculations off. I am going to range test the gun today at distances from 10y to 100y and track the actual path of the pellets. Then maybe some insight will be found. I’m also going to bring the chrono again and verify my speed. Again playing with theory to make the program match my shooting results a theoretical speed of 1200 has to be used to get the results I have found. I’m sure my fx t-12 in .22 is not shooting 15.9 jsb exact pellets at 1200. I’m going to bring all my test equipment (lap top, tablet, chrono, targets, range finder, gps, tape measure, ruler, whiskey) out and try to resolve my problem. I’m going to leave my email address as maybe it’s easier to share findings via email. Hope your day is great and thank you for your input & assistance.
    Scott Shepherd

  7. Hi Scott,
    Thanks for your quick reply. So, summarizing your problem; your 70 yards point of impact (POI) is 1 mil low where chairgun says it should be 2 mil. All with a 35 yard zero for a 22 cal airgun at about 30 fpe power with a scope height of 2 inch. This is a huge error.
    When I put this in Chairgun I need to set the zero at 56 yards to get a 1 mil low at 70 yards. But then the 35 yard trajectory is 1 mil high. You would have noticed that.
    To get a 35 yard zero and 1 mil low at 70 yards the scope height needs to be set to 4,5 inch ! I’m sure that woudl not have gone unnoticed as well.
    I’m very curious to hear about your findings at the range. Keep us informed. I’m sure there is an error somewhere. One of the chairgun inputs must be a long way off.
    Kind regards, Sven.

  8. Got a return call from a Hawke rep today about scope height. He conformed that the number to put into the software is the actual height of the center of the scope tube above the center of the barrel. So, measure scope diameter at one location. diameter of the barrel or shroud at the same location and the distance between the bottom of the scope tube and the top of the barrel or shroud. Davide the scope diameter by 2, divide the barrel or shroud diameter by 2 , measure the distance between the bottom of the scope tube at the same location and the top of the barrel or shroud at the same location and add the three numbers together. You now have “scope height”.

    • Hi George,
      Thanks for your input. But I have to disagree with the Hawke guy. The method of the Hawke representative that you mention will work and give you a result that is usually not far off. But every quarter inch error in scope height input will give a quarter inch error on your target (at any distance). A quarter of an inch (or 6,35mm) may not be a problem when shooting at a fixed distance on a paper target when you have the possibility to do sighting shots. But in Field Target shooting when shooting on a 15mm (0,60″) hitzone target at an unknown distance this may cost points in competition.
      Kind regards, Sven.

  9. Thanks Sven,
    It seems that you really have this software dialed in. I’ll go with your method. There is one element of your way I don’t understand. Won’t the POA and POI differ depending on how far away the paper target is from the end of the barrel? You said you were able to get in close, 1′, from the paper. If you had been out at 1.5′ the results would have been different. How can I know the distance I end up with is going to give me the correct scope height?
    Thanks,
    George

    • Hi George,
      Thank you for your reply. And yes, you are right. The distance from barrel end to paper target does matter. But half a foot (15cm) difference has only a very small, say minute, effect. The angle between line of sight and the centreline of the bore is very shallow. And at this shallow angle half a foot, or even a full foot, will make only a small difference.
      I use Chairgun (my old version 2) to determine the angle between boreline and sightline. At a scopeheight of 6,9 cm and a zero distance of 50 meters the boreline and sight line intersect at 10 meters distance from the muzzle. When I have done my trigonometry correctly this is an angle of 0,4 degrees. Half a foot more will not make a difference at this shallow angle.

  10. Thanks! One more question Should I measure from turret to actual end of barrel or shroud on a Benj Mrod to get the proper sight base?
    Thanks

    • Hi George,
      In my old Chairgun version 2, which I use mostly, the sight base input does not exist. In found it the new Hawke version. But it has only a minimal effect. When I click the plus and minus button to change the sight base distance I need to change the sight base by more than 5 inch to see the POI in clicks (1/4 moa) react. So the number for the sight base is not very sensitive.
      For Scope Height it is VERY sensitive. I only need to change 0,05 inch and I see a click difference at 10 yards. This proves my point that scope height is an important parameter in Chairgun. And you need to have it right to get good chairgun ballistic tables.

  11. Hi Sven,

    I just now discovered your site and used a modified version of your technique to discover that my scope height measurement was off by 0.1″ — I had measured it and set in Chairgun as 1.85″, but it was actually 1.75″.

    What I did was make a “wheel” with a wooden dowel “axle” taped to snugly fit the bore of my .22 air rifle. From the center axle point, I used a compass (the drawing kind) to make concentric circles from 1.5″-2.0″ at every 0.1″. Since the wheel was right at the end of the muzzle, I could not get a crisp picture, even with the pinhole and a bright light on the white surface, so even though I could see the rings, they were not sharp enough. I used painter’s blue tape to make horizontal tangent stripes inside the 1.5″ mark and outside the 2.0″ mark and then progressively taped in by 0.05″ until I saw the edge of the tape coming close to the horizontal crosshair, then I approached from the other side until I could just see a white line behind and centered vertically on the horizontal crosshair. The middle of that thin white stripe to the center point was my measurement — 1.75″.

    Thanks for posting your technique, and I hope my version of it is helpful to folks.

    Regards,
    Shawn

    • Hi Shawn,
      Thanks you for your post. Very useful to have another way to measure scope height. A picture of your setup would be great to show the readers what it looks like.
      Thanks, Sven.

      • Sven,

        I’ve already shot it as a target! 😛 It’s just concentric rings centered on and attached to the muzzle via an “axle” which is kind of like a regular boresight too. The use of tape should become obvious if anyone tries it.

        If I do it again, I’ll take pictures.

        Regards,
        Shawn

  12. Hi Sven, I tried your method, worked well with exception that the pellet hole was slightly off the vertical line, but measured 4.3cm for scope height. I assume that slight “Off centre” shot was a result of my hole in the foil not being exactly in the centre? Regards Geoff

    • Hi Geoff,
      Thanks for posting your experiences here.
      I don’t think an off centre hole would cause an off centre shot. I think it is either a parallax error or your scope is not in line with the barrel in the horizontal plane (cross over effect).
      Try 5 shots in a row to see if the problem is consistent.

      Good luck, Sven.

  13. Hi Sven,

    Merry Christmas! I am using your method to determine the scope height. The question that I have is: do I need to zero the scope first, or should I center the scope using the mirror method and then determine the height?

    Thank you, Jacob.

    • Hi Jacob,
      It does not matter much. The reticle is only moving a very small distance when you use this method because the paper is very close to the scope.
      And the scope height is a much larger parameter.
      Best regards, Sven.

      • Hi Sven,

        Thanks for your reply. I kind of played with it while taking multiple shots, and you’re right, it didn’t matter.

        Thank you for this great method!

  14. Mike Oxtinks

    I Chrono the pellet I’m going to use.

    I set my zero at 30m. I move the target out to 50m (2 sheets of A4) I have one or two vertical lines and one horizontal line near the top of the page.

    I shoot 3-5 pellets with cross hairs on the horizontal vertical cross on the target. I measure the drop in mm from POA to POI. I use “calculate BC from actual POI – which give BC)

    I enter the values of actual POI at 50m and zero data along with scope height (which I have always measured at the objective end/bell – Half scope dia + half barrel dia + distance to barrel from scope bell). I move the target to 10m – 15 – 20 – 25 etc and use the Chairgun app data to see if I POI’s/holdover/holdunder is correct by shooting the target at these distances. It’s usually right. (close enough for 10-45m pest control. Groups are usually well under a 5p piece.

    Chairgun is only as good as the data you enter.

    I spoke to Hawke re – BC changes to get the correct pellet curve for my field shots and they said “manipulate it ( Chairgun ) as you need to to ensure that out in the field the read out is correct” ie . . . where Chairgun says 1.75 mil dots holdover for a given shot. . . . . you do indeed make the shot and All shots using the inputted data.

  15. I like all this ‘science’ stuff and have/do use Chairgun myself.

    This thread has an underlying message. My character is to instinctively look to the ‘science’ behind something, it wasn’t until towards the end that I saw the underlying message. The focus on scope height hides that it is about massaging the inputs to Chairgun to get it to predict what we get when we shoot the gun.

    Having calculated the scope height (ala Tony’s method), measured the fps with the chronograph, and all the other parameters Chairgun needs, I confirm the predictions by setting up sheets of paper at 5yd steps and take shots.

    So why not shoot paper in the first place and forget about analogues?

    • Thank you Steve, that’s exactly what I do to make my range cards, and what I would advice everyone: Shoot paper at 2 to 4 meter intervals and record the turret settings.

      Chairgun has its use, shooters can really learn a lot from it, but the real world conditions are difficult to catch in a computer model such as Chairgun.
      One example: I recently found that two identical Sightron FT scopes reacted completely different to the adjustment of the parallax side wheel. One shifted the reticle 2,5 inch downwards between the 8 and the 50 meter setting. With this scope I need much less clicks to go from the 8 meter zero to the 50 meter zero than with the other identical scope. A collimator gave me the answer, but computer models like chairgun will be largely off when using this scope.
      The shifting scope is not giving me any trouble, it is very reliable and shifts the same amount every time in an exactly vertical line. (would it have shifted horizontally it would of course be useless and have needed repair).

      So I fully agree with you: Shoot and record to make your range card!

  16. Good point. There’s nothing like the science of actual, real field data. If you want to know where your point of impact is at 79 yards go set up a piece of paper at 79 yards.

  17. Hi Sven
    An interesting topic
    I was tidying the garage today and I have some 25 and 30mm o/d tube. If I took the scope off and put a length of tube the same diameter as the scope in the mounts and the tube was level with the end of the barrel and I then measured the distance from center of barrel to center of tube, do you think using this dimension would work as the scope height in chairgun.
    I’m asking as I don’t want to take off the scope if you don’t think this method will not be accurate

    • Hi Bob, that’s an interesting idea. In theory the centre line through the rings is not the same as the optical line through the scope. I don’t know how much it would differ, and if the difference is small (say under 5mm ) it would be no problem. If the difference would be larger it would be a problem. It would be nice to test and see.
      On the other hand, the aluminium foil test is easier because everything stays in place, not needing to take the scope of is a great advantage.
      I will check if I have any 30mm tube of sufficient length lying around. To make the comparison myself.
      Thank you for handling this good idea.
      Regards, Sven.

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