Chairgun

[Blackbaronfish]

How many people find that Chairgun isn't totally Bob on. Well it could be that your not inputting the correct figures.
When it asks for scope height it actually means line of sight and pellet path near to the muzzle.
See below as it just might make a difference.

https://airgunaccuracy.wordpress.com/ch ... pe-height/

BBF

[rhogg4696]

Why can't some smart person work this out from the scope height on the rifle, as we all know it, taking into account the distance from the muzzle back to the end of the scope? Surely there should be some formula. I am too old to remember trigonometry from school.

[mrb911no2]

Thanks BBF,

That explains alot - I was using the incorrect method!

Mark

[darran77]

Me doing it wrong too, i will now have to re do my measurments and check the results.
Thank you BBF
daz

[gdavison]

Why can't some smart person work this out from the scope height on the rifle, as we all know it, taking into account the distance from the muzzle back to the end of the scope? Surely there should be some formula. I am too old to remember trigonometry from school.

in the discussion for the article, some one posts a mathematical way to calculate the figure

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

[rhogg4696]

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)

I am not sure how you come about the "Distance scope objective to first zero point". For example I am zeroed at 50m so my first zero is around 12m and not anything like 1.5m.

[gdavison]

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)

I am not sure how you come about the "Distance scope objective to first zero point". For example I am zeroed at 50m so my first zero is around 12m and not anything like 1.5m.

Here is the comment from some one called Jo https://airgunaccuracy.wordpress.com/chairgun-and-scope-height/#comment-127 perhaps post the question to them ?

[gary martin]

this is how i do it. get a cover for the objective lens of your scope. drill a small hole approx 6mm dia in the center of it. put the magnification on its lowest setting. place a target cross about 1ft in front of the barrel, with the small hole you should be able to focus on the cross. shoot at the cross, then measure how much lower the point of impact is than the cross, this will be your scope height.
Gary

[Sake-san]

And even then, no substitute for checking it out in real time against targets at measured distances, from your principle shooting position...

[gary martin]

And even then, no substitute for checking it out in real time against targets at measured distances, from your principle shooting position...

Too true, the ballistic coeffiecient quoted may be different from barrel to barrel. Have to say though, chairgun is very close.