1000yd wind zero

[macguru]

I would think we are talking about coriolis, not spindrift, as its always to the right in australia, but i think any breeze at all would overwhelm this effect at these ranges, although it would be there ....
******
(Correction its left in aus)...

[shooter mcreid]

Macguru, are you saying the coriolis effect causes drift to the right in aus or have I interpreted incorrectly?

[pjifl]

Ballistics abounds with poorly defined terms - or even worse, terms that have taken different meanings over the years.

The 'crawling' of a RH twist projectile to the right at long range is real and probably best called 'Spindrift'. The mechanism is actually quite complex and many older texts explain it away wrongly but the shooter need not worry about that and simply know the effect.

Nearly all barrels these days are RH twist. The 303 was usually LH twist and quite odd so some older articles relating to 303's could be misleading today.

It does have an effect at long range in a very light twitchy fishtail wind which crosses zero. Otherwise it is usually masked by other factors.

Peter Smith.

[macguru]

oops no ....

coriolis is to the left in the sth hem. increases as you go away from the equator. its zero at the equator, max in tassie
also depends on compass bearing i think, ill look it up tomorrow
should only be a few inches though

[bsouthernau]

also depends on compass bearing i think, ill look it up tomorrow
should only be a few inches though

You're mostly correct - it is a matter of inches and the vertical component is bearing dependent. Horizontal component of coriolis depends only on latitude and speed of the bullet.

Barry

[bsouthernau]

Flat out at the moment loading ammo for Lower Light.Hope to catch up with the Match Rifle and Long Range F/class crew there. The plan is to shoot FTR in the Queen's,then follow on with Match Rifle.
Mike.

And I should be in the loading room too instead of gasbagging on the computer. Can't make it for the Queens this year - hoping to arrive some time on the 7th.

[williada]

Barry,

Based on Lower Light's altitude of 14.5 ft, mean October weather conditions at 3 pm with barometric pressure of 30.002 hg, 68 degrees F, relative humidity of 52 %, firing direction South, Latitude -34.54, muzzle velocity of 2690 fps, .308” 210 Smk, BC (G1) average .619, in a 1 in 10 barrel, you can expect using G1 BC 9.5 inches to the right drift and with G7 7.7 inches drift to the right.

This means your early estimate is pretty well spot on using G7. This means your 1000 yard zero is approximately ¾ minute left. Cheers.

[DaveMc]

Hi Barry, It should be you describing all this to us rather than the other way round but here goes my input anyway.

As presented in another thread previously.

A simple formula for horizontal component of coriolis effect is
(0.00007292 * (distance in feet)squared * sin (lat)) divided by average velocity (in fps) - answer is in feet so multiply by 12 for inches.

It is indeed to the left and (somewhat) counters a bit of the Right spin drift. For ball park figures in SA it is in the order of 2 to 2.5 inches (depending on time of flight). In cairns it is generally less than an inch.

Running same figures as williada above - I get a flight time of around 1.5 seconds
Coriolis effect of 0.00007292*3000*3000*sin(34.5)/2000 = 2.25 inches (approx

Right spin drift of 7.5 - 7.9 inches (similar to Williadas calculations above) countered by a corioils of around 2.25 brings you down to around half a minute.

Practically however I have found winding on 1/2 to 3/4 tends to be good enough and just add a click at each range from 500 back. (ie 300 wind zero, I add 1 click at 500, 700, 800, 900, 1000 or thereabouts and seems to work very well.

[bsouthernau]

Thanks all and yes Dave I'm comfortable enough with coriolis. My initial query was to confirm my hand calculations of the spin drift and - as happens on this forum - the thread has drifted a bit itself. I've since found that one of my ballistics programs does calculate the spin drift but only if I ask for output in inches - gives zeroes for output in moa.

I've also found that the calculator on the Applied Ballistics website is a ripper - only one I've come across that includes the crosswind effects on elevation.

Barry