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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
BALLISTIC COEFFICIENTS/Twist Rates
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<blockquote data-quote="LouBoyd" data-source="post: 419271" data-attributes="member: 9253"><p>As soon as you apply any "G coefficient" to a bullet reducing the information about the bullet to a single number (or even a few numbers) you have thrown away whatever useful information has been gathered about a specific bullet though testing. It's sort of like fitting clothing with the S M L XL XXL </p><p>designations but ignoring the variations of individual body parts. </p><p></p><p>G7 numbers are not inherently better than G1 numbers. They simply are a better but still limited model for slender boatail bullets typically used for long range shooting. G1 coefficients may be better for the bullets used in 100 yard benchrest and for small game hunting. Most 22LR bullets fit the G1 profile pretty well as do most bullets used for black powder rifles. </p><p></p><p>G1 and G7 coefficients are a handy way to compare bullets, and they are a useful marketing tool, but both are a detriment to precise ballistic calculations. What's is needed for precise ballistic calculations is to throw out all ballistic bullet models and simply measure (or calculate if there is sufficiently capable software developed) the actual drag curves for a standard atmosphere for each model of bullet made by each manufacturer in each caliber. All of that information would not fit in a practical size book, but it would probably fit on a single DVD ROM, or at least one from each manufacturer. The data would need to include a wide velocity range. It also needs to include stability information and the rate of spin decay vs velocity. There is presently no reliable method of calculating downrange bullet stability for a given bullet and atmosphere and determining it from shooting tests is very difficult. Existing published ballistic data is unreliable at transonic and subsonic velocities for any manufacturer that I'm aware of. Ballistic programs give answers you can't have any faith in it even when multiple BCs are given. Typical multi-BC data gives the lowest velocity BC number as covering "under 1800 fps" or similar. </p><p></p><p>What should a shooter do if they want an accurate drop chart for long range shooting with todays available data? The only answer I'm aware is not to rely on published BCs other than as a starting point. The only method is to use your own shooting tests instead. If you measure drop and lateral drift vs distance for several distinces over the range you shoot using your rifles and your loads along with measurements of the air density you can generate pseudo BCs which will give fairly accurate prediction of bullet drop and spin deflection. The existing ballistics programs are reasonably good for interpolation between the measured points and for adjusting for different air density. It's a lot of work and it will still have errors. Most ballistic programs have no model for handling bullet yaw, precession, and dampening so those errors will remain (and be more apparent) as air density changes from the conditions which existed when the shooting tests were made. The ballistic software simply has no code to account for those effects. Some software does exist (six degree of freedom models), but its usually impractical to obtain precise enough data in the input conditions to make the calculations more useful than the commonly used "McCoy" model. </p><p></p><p>I consider the term BC to mean "Before Computers" more than "Ballistic Coefficients". They were useful in the first half of the 19th century when ballistic calculations were done by hand using books of log and trig tables. Error correction (in the math) was handled by having multiple people do the calculations and comparing the results. At that time using a few simple ballistic bullet models made a lot of sense. But mathematical computation is no longer a limitation. Complete ballistic modeling can be done using a $300 PC from WalMart in less than a second. What's missing is the drag vs velocity measurements and stability data for each bullet from each manufacturer. Yes, that's a lot of data. Few manufacturers (any?) have it to publish. </p><p></p><p>A few manufacturers are giving G7 instead of G1 coefficients for bullets where the G7 is a better model in the velocity range most hunters and target shooters use. As long as people belive that "better" BCs are desirable bullet manufacturers will continue to publish them. Shooters seeking the highest accuracy are still free to ignore the published BCs.</p></blockquote><p></p>
[QUOTE="LouBoyd, post: 419271, member: 9253"] As soon as you apply any "G coefficient" to a bullet reducing the information about the bullet to a single number (or even a few numbers) you have thrown away whatever useful information has been gathered about a specific bullet though testing. It's sort of like fitting clothing with the S M L XL XXL designations but ignoring the variations of individual body parts. G7 numbers are not inherently better than G1 numbers. They simply are a better but still limited model for slender boatail bullets typically used for long range shooting. G1 coefficients may be better for the bullets used in 100 yard benchrest and for small game hunting. Most 22LR bullets fit the G1 profile pretty well as do most bullets used for black powder rifles. G1 and G7 coefficients are a handy way to compare bullets, and they are a useful marketing tool, but both are a detriment to precise ballistic calculations. What's is needed for precise ballistic calculations is to throw out all ballistic bullet models and simply measure (or calculate if there is sufficiently capable software developed) the actual drag curves for a standard atmosphere for each model of bullet made by each manufacturer in each caliber. All of that information would not fit in a practical size book, but it would probably fit on a single DVD ROM, or at least one from each manufacturer. The data would need to include a wide velocity range. It also needs to include stability information and the rate of spin decay vs velocity. There is presently no reliable method of calculating downrange bullet stability for a given bullet and atmosphere and determining it from shooting tests is very difficult. Existing published ballistic data is unreliable at transonic and subsonic velocities for any manufacturer that I'm aware of. Ballistic programs give answers you can't have any faith in it even when multiple BCs are given. Typical multi-BC data gives the lowest velocity BC number as covering "under 1800 fps" or similar. What should a shooter do if they want an accurate drop chart for long range shooting with todays available data? The only answer I'm aware is not to rely on published BCs other than as a starting point. The only method is to use your own shooting tests instead. If you measure drop and lateral drift vs distance for several distinces over the range you shoot using your rifles and your loads along with measurements of the air density you can generate pseudo BCs which will give fairly accurate prediction of bullet drop and spin deflection. The existing ballistics programs are reasonably good for interpolation between the measured points and for adjusting for different air density. It's a lot of work and it will still have errors. Most ballistic programs have no model for handling bullet yaw, precession, and dampening so those errors will remain (and be more apparent) as air density changes from the conditions which existed when the shooting tests were made. The ballistic software simply has no code to account for those effects. Some software does exist (six degree of freedom models), but its usually impractical to obtain precise enough data in the input conditions to make the calculations more useful than the commonly used "McCoy" model. I consider the term BC to mean "Before Computers" more than "Ballistic Coefficients". They were useful in the first half of the 19th century when ballistic calculations were done by hand using books of log and trig tables. Error correction (in the math) was handled by having multiple people do the calculations and comparing the results. At that time using a few simple ballistic bullet models made a lot of sense. But mathematical computation is no longer a limitation. Complete ballistic modeling can be done using a $300 PC from WalMart in less than a second. What's missing is the drag vs velocity measurements and stability data for each bullet from each manufacturer. Yes, that's a lot of data. Few manufacturers (any?) have it to publish. A few manufacturers are giving G7 instead of G1 coefficients for bullets where the G7 is a better model in the velocity range most hunters and target shooters use. As long as people belive that "better" BCs are desirable bullet manufacturers will continue to publish them. Shooters seeking the highest accuracy are still free to ignore the published BCs. [/QUOTE]
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