Index Home About Blog
From: (Bart Bobbitt)
Subject: Doping the Wind
Organization: Hewlett-Packard Fort Collins Site

In highpower competition, correcting for wind drift is often very challenging
and frustrating.  Here's some information that may help those who want to
improve their skills and knowledge regarding wind doping.  It'll also apply
to any medium to long range shooting situations while hunting, too.

A study of ballistic tables reveals that the longer a bullet is in flight,
the further it will move sideways in a cross wind (from 9- or 3-o'clock
relative to the line of fire).  I've found the wind drift at different
ranges for the same bullet/velocity combination has one, very interesting
common set of denominators: the numbers 1, 2, 3, 4 and 5.  And these numbers
are directly related to these respective ranges in the same order: 300, 600,
800, 900 and 1000 yards.

This `by the numbers' system works as follows as the range increases:

  1.  For a given relative crosswind, the mirage (heat waves) will wrinkle
      across your spotting scope's field of view at some rate.  It's really
      important that you are able to remember from day to day (match to
      match) what rate is worth a given amount of correction at each range.

  2.  At 300 yards for a given crosswind speed, whatever the windage
      correction is (MOA or clicks on your sight), they represent the number

  3.  At the next and succedingly longer ranges of 600, 800, 900 and 1000
      yards, the same crosswind speed (or rate the mirage wrinkles across
      your scope's field of view) will deflect that same bullet/velocity
      combination an amount equal to the MOA or clicks used at 300 multiplied
      by the base number for the longer range.  For example, if a 2 MOA
      correction was needed at 300 yards, the correction at longer ranges
      would be:

         * 2 times as much at 600 yards, or 4 MOA.

         * 3 times as much at 800 yards, or 6 MOA.

         * 4 times as much at 900 yards, or 8 MOA.

         * You're right; 5 times as much at 1000 yards; 10 MOA.

This system also works very well in reverse.  For example, if you need to
to use 25 clicks of windage correction at 1000 yards and will next be
shooting at 600 yards, you would:

  1.  Divide the 600-yard base number (2) by the 1000-yard base number (5)
      and get 0.4; four tenths.

  2.  Multiply the sight correction used at 1000 yards (25 clicks) by the
      answer above (.4) to get 10 clicks; the amount of wind correction
      needed at 600 yards for the same crosswind speed.  Note that the
      600-yard base number (2) and the 1000-yard base number (5) have the
      same respective ratio as the windage correction at 600 yards (10 clicks)
      to the correction at 1000 yards (25 clicks).

Some folks may chose other algorithms using the same numbers to get the right
answer.  That's fine; what ever works mentally best for you.

With any rule-of-thumb system, there's gonna be some errors.  And this one
is no exception.  But the maximum error I've noted in comparing ballistic
table numbers with actual on-the-range-in-the-wind experiences, the error
won't be more than 10%.  So, if you determine you gotta have a 15 minute
correction at 1000 yards with a strong wind makin' those range flags point
straight out, you might be plunking your first shot in the 9 ring to one
side.  But hangin' a 9 for your first shot in a 17 mph cross wind ain't too
shabby at all!

What about 200 yards?  The 200-yard base number is one-half; 0.5.  Wind
corrections at 200 yards for a given wind are just about exactly half the
MOA/clicks of what's used at 300 yards for the same crosswind speed.


From: (Bart Bobbitt)
Subject: Re: mirage
Organization: Hewlett-Packard Fort Collins Site

For any given mirage (heat waves) rate it moves across your spotting
scope's field of view, at each range there is a given amount of
windage correction needed.  It's typically more in the cool mornings,
but a bit less in the hotter afternoons.  I posted an article on
doping the wind some time ago.  That information applies here.  What
I do is first look at the `wrinkle rate.'  Then estimate how much
windage correction is needed.  Finally, I shoot and note where I called
the shot, then note where the spotter is relative to where I called
the shot.  If my estimate was in error, I note what the correct windage
is for that amount of wrinkle rate and make a mental note of it.  If
the wrinkle rate increases by 25%, then I need to put on 25% more wind.
If it reduces by half, then I cut my windage correction by 50%.

It's important to look at the wrinkle rate at the top of the target.
Remember, your bullet is above the line of sight for 95% of its time
in the air.  Sometimes, range flags will help tell you what the wind
is doing, but only if they are 100% cotton flag bunting, 15 or so feet
long and dry.  Other flag sizes and materials just don't work too well.
And only watch the upwind flags; those down wind from where you are
tell you what has happened and you can't correct for history.  If an
upwind flag changes its behavior, you'ld better change yours by adjusting
your windage to correct.


From: (Toby Bradshaw)
Subject: Re: mirage
Organization: University of Washington, Seattle

In article <> (Bart Bobbitt) writes:
#Toby Bradshaw ( wrote:
#: In the latest benchrest match I shot, there
#: was pretty severe mirage at 200 yards, enough to move the crosshairs
#: from 9-ring to 9-ring.
#This is interesting.  Having anchored a scope down on a range, focused
#at a 300-yard target, then checking its reticule position on the target
#as the heat waves (mirage) build up during the day and later go away
#in the evening, I did not notice any shift of the reticule relative to
#the target's center.

I was looking through the scope on a rail gun shooting groups of <0.500"
at 200 yards, and the target image is distorted enough to move the
crosshair out of the 10-ring (1" diameter).  If there were no distortion,
the mirage would be invisible anyway :)

#If this really happens, then surveyors sure have a lot of bad data;
#they'd better not do any sighting on their stadia rods or 3-corner
#reflectors except at night, in the wintertime, when there ain't any
#heat waves.

One can easily determine the true center of the target (at least when
the mirage is not severe that it can't be seen at all) by careful
observation, and I assume surveyors can do the same thing.  The
image moves around in the mirage, but returns the same spot when
it "snaps back".  In heavy mirage, you can't see your bullet holes
even at 200 yards, because the image is so distorted.  Beats me
what surveyors do then.  Probably look at something bigger than
a bullet hole :)

-Toby Bradshaw

Index Home About Blog