Wind Drift

[camies]

Can anyone help (and yes I know that wind drift is an oldie, thrashed to death subject). Recently I've been introduced to the world of long distance (100 yards) target shooting with PCP air rifles.

Despite hours of searching on the web, all the articles I've read and software I have found cannot cope with the slow speeds (M.V.'s round 500fps) and the diablo shape (BC typ. 0.035) of the airgun pellet (typ. 21gr).
Plenty of programs accept the parameters but the results are wildly inaccurate.

So, the basic questions are:-
In G1,G2 etc terms, where does the diablo shape fit in?
Is there any software (freeware) SPECIFICALLY for airgun pellets?
or, does anyone know the formulas involved to allow me to write my own range cards.
Please note that I'm not a maths major so any maths answers need to be in K.I.S.S. (Keep it stupidly simple) terms so my eight year old can type them into the Excel program for me!

About the only thing I know for certain is that doubling the distance DOES quadruple the wind drift (ish) and 22LR low power rounds don't fly the same as airgun slugs!

Any help appreciated.

[Hunter_zero]

Can anyone help/Any help appreciated.

Have a look here : http://www.eskimo.com/~jbm/software/software.html

Bount the best on the net.

John

[camies]

Thanks John, already played with their calcs but even they come unstuck because (I think) because of the unusual diablo shapes. It seems that airgun slugs don't follow standard ballistic formula.

[simmmo]

Have you tried Chairgun?

Andy

[Harry's Lad]

Can anyone help (and yes I know that wind drift is an oldie, thrashed to death subject).

Perhaps a quick review of those thrashings would reveal Dr F. W. Mann’s classical formula:

D = 12 * W * Sin(A) * (T - (3 * R / V )) …. (Inch)

where:
D = Wind Deflection (Inch)
W = Wind Velocity (Ft/s)
A = Angle between the Wind Velocity Vector and the trajectory
T = Transit time between muzzle and target (sec)
V = Muzzle Velocity (Ft/s)
R = Distance between muzzle and target (Yard)

We also know that the transit time (T above) can be found from:

T = 24000 * BC * (Exp(R / (8000 * BC)) - 1) / V …. (second)

where:
BC = Ballistic Coefficient

So, substituting T in the first equation:

D = 12 * W * Sin(A) * (24000 * BC * (Exp(R / (8000 * BC)) - 1) / V - (3 * R / V )) …. (Inch)

Plenty of programs accept the parameters but the results are wildly inaccurate.

By ‘wildly inaccurate’ do you mean that the results of various ballistic software disagree with each other or with your experience?

So, the basic questions are:-
In G1,G2 etc terms, where does the diablo shape fit in?

I’ve done a bit of work in evaluating a G* curve for waisted airgun pellets over the years but, over the limited range of velocities available and within the accuracy of generally available chronometers, said curve would seem to be essentially flat. i.e., the Ballistic Coefficient remains at much the same value between 0.2 and 0.8 Mach. In any event, the shape of the pellet’s ‘G’ curve has no discernable effect on the wind-drift.

Is there any software (freeware) SPECIFICALLY for airgun pellets?

ChairGun2

… or, does anyone know the formulas involved to allow me to write my own range cards.
Please note that I'm not a maths major so any maths answers need to be in K.I.S.S. (Keep it stupidly simple) terms so my eight year old can type them into the Excel program for me!

See above.

Alternatively, have a look at the WindCalc calculator

HTH
Dave

[camies]

Wow, Harrys Lad.

Maths and all! Ok, If I can get my genius son to type the formula:-

D = 12 * W * Sin(A) * (24000 * BC * (Exp(R / (8000 * BC)) - 1) / V - (3 * R / V )) …. (Inch)

into Excel and get it to run, I'll give it a go. My slide rule is a bit ancient to cope let alone me understand Exp functions. (remember, I'm no Maths major).

"wildly inaccurate" means that practical tests (using a wind speed meter and a chronograph) over 25 yards don't translate to 50 yards very accurately. Winds of +7mph at 25yards, give about 1 and 2/3 calculated value at 50 yards.
Go beyond that and only by shooting over water can I see where the POI is and that is difficult to gauge when translating splashed POI at 50yards. the best I can do at that range is using mildot measurement.

I find it quite easy to hit a target accurately on a windless day at 100 yards AND I can easily cope with slope dope but, even chairgun2 struggles at range.
windcalc was the closest, close enough to draw up a guess-timate range card anyway and with field trials and a pencil adjusting values worked quite well out to 50 yards beyond that-- hopeless. It's a bit difficult going out each day to the same spot hoping for a constant wind a few MPH more or less than 'yesterday' though still, plenty of time as summer is coming!

Thanks for your input.