Working out the wind ?

Ammoman

Does anyone here have a method of working out how much windage to allow at a given distance with a given velocity of wind using fullbore rifles

say 20mph wind from 9 oclock with a 6.5 mm lapua 139gr bullet shooting at 650 yds ?

The elevation on the scope is 14.5MOA but the windage has me stuck?????

jaycee

Hi Ammoman...

You can download the ammo selector from the federal site

http://www.federalcartridge.com/default.asp?br=1

It's pretty small ..but it does include all the basic variables ,
Altitude, Temperature, Crosswind speed, distance , and Zero

(I make the answer to your question about 82inches of drift @ 650 yds
using a 140gr bullet with a muzzle velocity of 2600fps ..)

1 MOA of windage at 650 Yds would be roughly 6.5 inches
so divide the total drift (82 ) by 6.5 = 12.62
Apply this figure to the windage adjustment of your scope remembering
that each click is usually 1/4 MOA so 12.62 x 4 = 50 clicks. (Ish)

Based on a Federal 6.5x55 Swedish cartridge .

Also roughly because I had to guess the calibre, ballistic coefficient , temp and altitude.

Other ballistics programs are out there .. have a go on your "puter ..then compare results at the range ..

Ammoman

jaycee wrote:

Hi Ammoman...

You can download the ammo selector from the federal site

http://www.federalcartridge.com/default.asp?br=1

It's pretty small ..but it does include all the basic variables ,
Altitude, Temperature, Crosswind speed, distance , and Zero

(I make the answer to your question about 82inches of drift @ 650 yds
using a 140gr bullet with a muzzle velocity of 2600fps ..)

1 MOA of windage at 650 Yds would be roughly 6.5 inches
so divide the total drift (82 ) by 6.5 = 12.62
Apply this figure to the windage adjustment of your scope remembering
that each click is usually 1/4 MOA so 12.62 x 4 = 50 clicks. (Ish)

Based on a Federal 6.5x55 Swedish cartridge .

Also roughly because I had to guess the calibre, ballistic coefficient , temp and altitude.

Other ballistics programs are out there .. have a go on your "puter ..then compare results at the range ..

Thanks for that , I have a few different programs but they are not accurate , I know that there will be variations in the wind throughout the flight of the bullet , but I want to find out if there is a formula to calculate wind drift , obviously there is , thats how the Ballistics programs work , but what is it ?

gouda

Ammoman wrote:

Thanks for that , I have a few different programs but they are not accurate , I know that there will be variations in the wind throughout the flight of the bullet , but I want to find out if there is a formula to calculate wind drift , obviously there is , thats how the Ballistics programs work , but what is it ?

This is what you are looking for.




How a Ballistics Program Works
-or how to calculate a trjectory chart-
by Russ White
Up front I should say that this information is based on how The Shootist, version 2.21, works, and there are other ways of obtaining the information contained in a firing table. I should also set up some definitions to keep the confusion down.

A ballistic table generally does not contain information about a particular projectile's trajectory, but rather general information on the flight of projectiles. The table is then used in conjunction with a set of formulas to produce the trajectory information. Generally, a trajectory chart is computed in the following way:

Find the remaining velocity and time of flight to the range in question.
Find the drop of the projectile using this information.
Correct for the zero of the firearm in question.
Part I: Remaining velocity and time of flight.
A table and formulea for using it are included in the file Ingall.txt which may be printed out for convenience in use. This table and its formulea are derived from the Ingall's ballistic tables. These tables were founded upon the test firing of some 'standard' projectiles, and the resulting calculations must be corrected with 'ballistic coefficients.' This is simply a measure of the difference in air resistance between the 'standard projectile' used in compiling the table and an ordinary, common, bullet.

Why is a table used instead of a straight formula? Because it was found by many scientist doing ballistics test that a bullet does not lose speed uniformly. The rate of velocity loss is dependant upon a combination of many factors making the formulea for computing velocity loss very difficult and complicated. The main factor is the sound barrier; bullets lose velocity at a different rate above and below it.

Part II: Find the drop of the bullet.
This is done also on a table, but can be accomplished with formulas. The table used in The Shootist is once again included in the file Ingall.txt. It is based upon the premise that all objects fall at a given rate:


2
Drop (inches) = 193 x (time of flight)

Inherent inaccuracies occur, however, in the formula because of wind resistance, which actually works to slow down the fall of the bullet. The action of the air has already been computed into the table. Instructions on the tables usage are also included. An explination of the formulea involved in calculating the effect of the air resistance can be found in "Exterior Ballistics" by McShane (and others). The table The Shootist uses is one shown in "Hatcher's Notebook," which is derived from the one in "Exterior Ballistics. To show how much difference there is when the air resistance is figured in, the following comparison is made:


Yards 100 200 300 400

With 1.8 7.5 17.7 33.6
Without 1.93 7.72 19.76 37.36

This is based on a bullet beginning at 3200 fps with a ballistic coefficient of .44. The 'With' row was calculated using the table provided in Ingall.txt, while the 'Without' was calculated with the formula given above.

Part III: Correct for the sighting in of the firearm.
This involves a bit of trigonometry that will not even feel like trigonometry:

H is the height of the sight above the bore
D is the drop in inches of the bullet at sight-in range
S is the sight in range in inches
R is the range in question in inches
I is the drop from the bore at the range in question
T is the difference in inches from the line of sight

If the range in question is less than the sight in range
T = (D-I) - ((S-R) x ((D+H)/S)

If the range in question is more than the sight in range
T = (D-I) + ((R-S) x ((D+H)/S)




The sight in range is not covered by either of the two formulea becuase it is known to be 0.

For example:

A bullet fired from a gun sighted in at 200 yards drops 4.2 inches in its first 100 yards of travel. How many inches is it above the line of sight? The drop from the bore at the sight in range is 7.5 inches (these are calculated from the tables).

T = (D-I) - ((S-R) x ((D+H)/S)
T = (7.5-1.8) - ((7200-3600) x ((7.5+1.5)/7200)
T = 5.7 - (3600 x .00125)
T = 5.7 - 4.5
T = 1.2 inches (above the line of sight)




Another:

The same bullet drops 17.7 inches by 300 feet. How far off the line of sight is it now?

T = (D-I) + ((R-S) x ((D+H)/S)
T = (7.5-17.7) + ((10800-7200) x ((7.5+1.5)/7200)
T = -10.2 + (3600 x .00125)
T = -10.2 + 4.5
T = -5.7 inches (below the line of sight)




Other Formulas:

2
Kinetic Energy = ((Bullet wght/225218) x Velocity )/2

Energy Transfer = Kinetic Energy x Caliber

3 2
Optimum Game Weight = Velocity x Bullet Weight x 1.5012 x 10e-13

Mach = Velocity/1127

Miles per hour = (Velocity x 3600)/5280

Wind drift:
t = time of flight in seconds
s = wind speed in mph
a = angle of wind off trajectory path
v = muzzle velocity in feet per second
r = range in yards

Wind drift = (s x sin(a)) x (t - (v/r)




I know all this is a bit complicated, but with a little practice it becomes rather easy to make the computations necessary. For further information read "Hatcher's Notebook," which covers a lot of other interesting territory besides exterior ballistics.




Copyright © 1998-2003, Games++ All rights reserved. | Privacy Policy

When you say the programs you use are not accurate,how far off are they and have you checked the velocity of the bullet used with a chronograph?

jaycee

The attached file may help ..
It is in Excel format and you may be able to access the actual formulas used.

Either way it is pretty good ..instructions are included.

Ammoman

gouda wrote:

This is what you are looking for.




How a Ballistics Program Works
-or how to calculate a trjectory chart-
by Russ White
Up front I should say that this information is based on how The Shootist, version 2.21, works, and there are other ways of obtaining the information contained in a firing table. I should also set up some definitions to keep the confusion down.

A ballistic table generally does not contain information about a particular projectile's trajectory, but rather general information on the flight of projectiles. The table is then used in conjunction with a set of formulas to produce the trajectory information. Generally, a trajectory chart is computed in the following way:

Find the remaining velocity and time of flight to the range in question.
Find the drop of the projectile using this information.
Correct for the zero of the firearm in question.
Part I: Remaining velocity and time of flight.
A table and formulea for using it are included in the file Ingall.txt which may be printed out for convenience in use. This table and its formulea are derived from the Ingall's ballistic tables. These tables were founded upon the test firing of some 'standard' projectiles, and the resulting calculations must be corrected with 'ballistic coefficients.' This is simply a measure of the difference in air resistance between the 'standard projectile' used in compiling the table and an ordinary, common, bullet.

Why is a table used instead of a straight formula? Because it was found by many scientist doing ballistics test that a bullet does not lose speed uniformly. The rate of velocity loss is dependant upon a combination of many factors making the formulea for computing velocity loss very difficult and complicated. The main factor is the sound barrier; bullets lose velocity at a different rate above and below it.

Part II: Find the drop of the bullet.
This is done also on a table, but can be accomplished with formulas. The table used in The Shootist is once again included in the file Ingall.txt. It is based upon the premise that all objects fall at a given rate:


2
Drop (inches) = 193 x (time of flight)

Inherent inaccuracies occur, however, in the formula because of wind resistance, which actually works to slow down the fall of the bullet. The action of the air has already been computed into the table. Instructions on the tables usage are also included. An explination of the formulea involved in calculating the effect of the air resistance can be found in "Exterior Ballistics" by McShane (and others). The table The Shootist uses is one shown in "Hatcher's Notebook," which is derived from the one in "Exterior Ballistics. To show how much difference there is when the air resistance is figured in, the following comparison is made:


Yards 100 200 300 400

With 1.8 7.5 17.7 33.6
Without 1.93 7.72 19.76 37.36

This is based on a bullet beginning at 3200 fps with a ballistic coefficient of .44. The 'With' row was calculated using the table provided in Ingall.txt, while the 'Without' was calculated with the formula given above.

Part III: Correct for the sighting in of the firearm.
This involves a bit of trigonometry that will not even feel like trigonometry:

H is the height of the sight above the bore
D is the drop in inches of the bullet at sight-in range
S is the sight in range in inches
R is the range in question in inches
I is the drop from the bore at the range in question
T is the difference in inches from the line of sight

If the range in question is less than the sight in range
T = (D-I) - ((S-R) x ((D+H)/S)

If the range in question is more than the sight in range
T = (D-I) + ((R-S) x ((D+H)/S)




The sight in range is not covered by either of the two formulea becuase it is known to be 0.

For example:

A bullet fired from a gun sighted in at 200 yards drops 4.2 inches in its first 100 yards of travel. How many inches is it above the line of sight? The drop from the bore at the sight in range is 7.5 inches (these are calculated from the tables).

T = (D-I) - ((S-R) x ((D+H)/S)
T = (7.5-1.8) - ((7200-3600) x ((7.5+1.5)/7200)
T = 5.7 - (3600 x .00125)
T = 5.7 - 4.5
T = 1.2 inches (above the line of sight)




Another:

The same bullet drops 17.7 inches by 300 feet. How far off the line of sight is it now?

T = (D-I) + ((R-S) x ((D+H)/S)
T = (7.5-17.7) + ((10800-7200) x ((7.5+1.5)/7200)
T = -10.2 + (3600 x .00125)
T = -10.2 + 4.5
T = -5.7 inches (below the line of sight)




Other Formulas:

2
Kinetic Energy = ((Bullet wght/225218) x Velocity )/2

Energy Transfer = Kinetic Energy x Caliber

3 2
Optimum Game Weight = Velocity x Bullet Weight x 1.5012 x 10e-13

Mach = Velocity/1127

Miles per hour = (Velocity x 3600)/5280

Wind drift:
t = time of flight in seconds
s = wind speed in mph
a = angle of wind off trajectory path
v = muzzle velocity in feet per second
r = range in yards

Wind drift = (s x sin(a)) x (t - (v/r)




I know all this is a bit complicated, but with a little practice it becomes rather easy to make the computations necessary. For further information read "Hatcher's Notebook," which covers a lot of other interesting territory besides exterior ballistics.




Copyright © 1998-2003, Games++ All rights reserved. | Privacy Policy

When you say the programs you use are not accurate,how far off are they and have you checked the velocity of the bullet used with a chronograph?

Excellent , i will try it out this weekend and let you know how it works .

gouda

Ammoman wrote:

Excellent , i will try it out this weekend and let you know how it works .

Remember,it won't be much help if you do not have an accurate muzzle velocity confirmed by a chronograph.