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Old 11th September 2013, 09:29 AM
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Brian.Samson Brian.Samson is offline
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Join Date: Jun 2009
Member of: Pontefract, Doncaster Airgun Range
Location: Doncaster
Posts: 2,332

Originally Posted by raygun View Post
Chairgun scope height.

Mark a cross on something like a Yellow pages.
Set your scope to minimum PA adjustment.
Cover the Objective lens with tinfoil and poke a small hole (3mm) in the centre of the foil.
Sight the cross on the yellow pages and with your muzzle as close as possible to the yellow pages fire a pellet.
You can now measure from the centre of your pellet hole to the cross you've marked.
This will give you your scope height for Chairgun.

Originally posted by Harry's Lad, the author of Chairgun.

Hope this helps.

Yup, that's the correct way - Dave (Harry's lad) was actually quoting Steve_in_NC from the yellow forum, not sure where Steve's info came from, but I know that the calculations for Chairgun came from Steve (and they're actually available as open source).

I've had discussions with people in the past who insist that they're using mega calibrated verniers yada yada to measure the exact physical scope height, but this will almost always be wrong. No matter how accurate your ruler is, it will be wrong.

I even drew a little diagram to illustrate why it's going to be wrong. > Here <

The other factor that people can get wrong, especially with regards to clicks, is they assume that a turret marked up as 1/8" clicks, are actually accurate. They aren't accurate quite a lot of the time. There is a method you can use to find out what the real figure is, but like all of these measurements it can end up being more of a hassle to get the correct inputs for ballistic programs than it would be to just pin up bits of paper at all the distances you're interested in and shoot groups.

That would always be my recommendation - shoot groups on bits of paper if you want the correct answers. Ballistic calculators are useful for 'what if' scenarios, but I wouldn't trust them to give me the clicks for my turret before a competition.

I did a little bit of testing a few weeks ago with my TX200 and found that the measured BC figure for the batch of AA Fields I've got is between 0.020 and 0.025 at 55 yards. The measured BC for a batch of Daystate selects I have was 0.025 to 0.027. But using 0.024 will get you very close (if all the other measurements are correct, or close to being correct)
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