Advice on calculating Hold-over/under mil dot distances

[cooper_dan]

It uses a profile for a projectile shape. Usually "GA" which gives it a curve of how the BC is likely to change with drag/velocity (it's by no means a straight line).

That rings a bell. I think I used G1? for my springer. I might try to create a user profile from test data, but I bet it would change again as soon as the weather warms up

[Brian.Samson]

That rings a bell. I think I used G1? for my springer. I might try to create a user profile from test data, but I bet it would change again as soon as the weather warms up

Definitely don't use G1 - G1 is the form factor for a flat based bullet shape - G7 is a boat tail bullet.

GA in Chairgun is a custom profile that's not a bad match for a traditional waisted diablo shape. Maths is only a waste of time if you don't understand it

[bullbarrel]

Maths is only a waste of time if you don't understand it

It got Matt Damon all the way back from Mars

[GPConway]

All that adds up to a whole heap of errors and assumptions - and it's the reason why people will often say that Chairgun is wrong. It can only be as accurate as the information it's being fed - Garbage In, Garbage Out.

I'll see all of your assumptions listed and raise you at least half a dozen more.
The 'Garbage In, Garbage Out' thing is (has always been) very true although I've seen it rephrased as 'Warning: The accuracy of this software will depend entirely on the quality of your assumptions.'

Another point I forgot to mention in my earlier post. I know Chairgun works for some people, but for me it absolutely will not correlate with my HFT setup.
I don't know how chairgun deals with BC at different distances as the pellet slows down. Maybe someone cleverer than me will say. But if I used the chairgun holdovers for this gun, I wouldn't hit anything past 35 yards.***
But my S410 in 0.22 matches up perfectly.***

The Ballistic Coefficient doesn't vary with velocity (or temperature or pressure or humidity or moon-phase or a plethora of other things)...

BC = Sd/FF
Where BC = Ballistic Coefficient
Sd = Sectional Density = Weight (Lb) / Diameter² (inch²) and
FF = Form Factor = observed Cd / Reference Cd at any particular velocity.

... note that, for any particular projectile, Sd is constant and FF is also constant if (and only if) the correct reference drag law is used. Obviously, if Sd and FF are both constant then BC must be constant too. As Brian mentioned elsewhere in this thread, the GA drag law is probably the best fit for round-nosed, diabolo JSB Exact pellets and their close clones.
If you see curves of BC varying with velocity (common on some American websites the authors of which should really know better) then either the drag law has been badly chosen or, more likely, a (easy to use but wildly inaccurate) constant Cd model has been used.
*** See the earlier remarks re. Quality of assumptions ...

That rings a bell. I think I used G1? for my springer. I might try to create a user profile from test data,

Good luck with that (unless you've got access to a wind-tunnel and/or a Doppler Radar set). I'm told that Chairgun's developer took 8 months and well over 2000 data points to generate the GA curve. Not a simple task.

... but I bet it would change again as soon as the weather warms up

As above. Temperature dependence would indicate that your drag law is incorrect.
However, you can easily use Chairgun's environmental variables to match the the current atmospherics to re-calibrate the predicted trajectory to reality.
*** See the earlier remarks re. Quality of assumptions ...


George

[Rickenbacker]

It got Matt Damon all the way back from Mars

External ballistics is no laughing matter, Andy.