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Old 9th November 2013, 05:40 PM
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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: 2,330

Originally Posted by PaulD View Post
As Conor says you can only be certain of the click settings for your particular setup by shooting at paper! Chairgun will give you a starting point but I've found that two things really affect it's accuracy. 1) do not use the BC given, set two chrono's up one in front of the muzzle and one down range (ideally at 55yds) then work out the BC for your particular pellet/barrel. 2) if you are using a sloped rail where do you measure the scope height? I usually measure in the middle and then tweak it to suit. Why fanny about with all that when it's easier to do it properly I repeat the only way to be certain is on paper on a calm day!
Ey up Paul, I've been doing a fair bit of investigation into the trajectory calcs and Chairgun lately. I should say I totally agree that shooting at paper is a far more accurate method of getting your clicks right.

But... if you want know how to get Chairgun to closely match real world settings there are a couple of potential problems with your methods.

The first problem is - don't use the Chairgun function to calculate BC from two chrono readings - it will get the answer wrong. The reason it gets it wrong is a bit complicated to explain, but basically it doesn't take all the factors into consideration for getting the correct BC to use later on in the trajectory calculation. You can prove this for yourself. The most reliable method is to change the BC value in the Chairgun trajectory calc until the calculated velocity at the distance of your 2nd chrono matches your real world finding.

The second problem is - don't measure the scope height.. you don't want to know the scope height, you want to know the height of the line of sight above the muzzle.

The best method of doing this is to draw a cross on a bit of card, place the card right on the muzzle of your gun, point your crosshairs at the cross on the card and pull the trigger (being careful about ricochets etc.)

The only problem you'll find with this is that you won't be able to focus your scope on the cross because it's too close to your scope. To solve that problem, turn the mag down as low as you can, set the parallax as low as you can, then get a bit of tin foil and secure it over the objective lens of your scope with a rubber band. Now make a small hole in the centre of the tin foil effectively reducing the objective lens size to say.. 3mm. That should bring the depth of field down low enough so you can see the cross in your scope.

Once you've done that, just measure the distance between the cross and the hole you've just made in the card by shooting it on your muzzle.

That method will give you the correct line of sight height measurement to use in ballistic calculators and it will automatically take into account sloped mounts or packed mounts etc.
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