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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
my drop chart doesn`t match at 500yds and beyond?
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<blockquote data-quote="Warren Jensen" data-source="post: 20804" data-attributes="member: 21"><p>Lets talk theoretical here. I don't know if this is what is happening to Brent's bullets and trajectory, because I don't have enough information, yet. But, this is typical of what happens during long range bullet flight.</p><p></p><p>A bullet that is not fin stabilized requires spin, or rotation, to attain gyroscopic stability. The amount of spin necessary to keep the bullet point nose forward can be very precisely calculated. When insufficient spin is imparted to the bullet, and it has any type of ogive and not a wadcutter, the oncoming air will cause the overturning moment to overcome the gyroscopic stability and the bullet will tumble nose high over backwards. It will continue to tumble. When the spin is just sufficient to stabilize it, the bullet will fly with it's longitudinal axis directly into the oncoming air. When the spin is more than sufficient the bullet will start to precess and yaw so that it's longtudinal axis is high and to the right(for right hand twist) of the oncoming air, and the nose will be drawing little circles in the air. The rate of spin that the bullet has that is more than the minimum necessary to keep it stabile will cause this precession and yaw to increase.</p><p></p><p>Precession and yaw decrease the projectile's ballistic efficiency, the drag goes up and the effective BC goes down.</p><p></p><p>Rotational drag is always less than linear drag, except in the case of bullets designed for Balanced Flight. The bullets we are talking about here will always have less rotational drag than linear drag. That means that the spin rate will decrease less rapidly than the velocity. Consequently, as flight time increases the bullet will be continuing to spin faster and faster than is necessary for it to maintain gyroscopic stability. Hence, the precession and yaw will continue to increase. The drag will be increasing and the BC will be going down. This is called BC decay and is very common for bullets that are not either designed for long range flight or are overspun at the muzzle. Sometimes this is referred to as "overstabilized", which is a misnomer, because as this condition continues the yaw will become so exaggerated that the bullet will become unstabile and tumble. What it is doing is spinning too fast. It also is why only properly designed long range bullets will decelerate back through transonic in a stabile and predictable manner. The collapsing shock waves of transonic phenomena will destabilize any bullet with exaggerated yaw.</p><p></p><p>Very, very few bullets exhibit constant drag coefficients throughtout their flight. The concept that these BC variances fall into velocity brackets is an oversimplification of the physics involved, and leads to common mistakes in trajectory calculations.</p><p></p><p>Most manufacturers (myself included) publish BCs that accurately describe the bullet's flight at the muzzle. It is the most accurate and predictable method. To accurately predict your bullet's trajectory from your rifle I would need to know not only the true velocity and twist rate, to the tenth of an inch, but the engraving characteristics, # of lands, depth, etc., and the muzzle exit yaw numbers. </p><p></p><p>Or you can shoot your rifle at the various ranges, using the published BC as a reference, and see where the bullet is actually flying. This is the best way to determine what your rifle, with your load, is doing.</p><p></p><p>[ 06-13-2001: Message edited by: Warren Jensen ]</p></blockquote><p></p>
[QUOTE="Warren Jensen, post: 20804, member: 21"] Lets talk theoretical here. I don't know if this is what is happening to Brent's bullets and trajectory, because I don't have enough information, yet. But, this is typical of what happens during long range bullet flight. A bullet that is not fin stabilized requires spin, or rotation, to attain gyroscopic stability. The amount of spin necessary to keep the bullet point nose forward can be very precisely calculated. When insufficient spin is imparted to the bullet, and it has any type of ogive and not a wadcutter, the oncoming air will cause the overturning moment to overcome the gyroscopic stability and the bullet will tumble nose high over backwards. It will continue to tumble. When the spin is just sufficient to stabilize it, the bullet will fly with it's longitudinal axis directly into the oncoming air. When the spin is more than sufficient the bullet will start to precess and yaw so that it's longtudinal axis is high and to the right(for right hand twist) of the oncoming air, and the nose will be drawing little circles in the air. The rate of spin that the bullet has that is more than the minimum necessary to keep it stabile will cause this precession and yaw to increase. Precession and yaw decrease the projectile's ballistic efficiency, the drag goes up and the effective BC goes down. Rotational drag is always less than linear drag, except in the case of bullets designed for Balanced Flight. The bullets we are talking about here will always have less rotational drag than linear drag. That means that the spin rate will decrease less rapidly than the velocity. Consequently, as flight time increases the bullet will be continuing to spin faster and faster than is necessary for it to maintain gyroscopic stability. Hence, the precession and yaw will continue to increase. The drag will be increasing and the BC will be going down. This is called BC decay and is very common for bullets that are not either designed for long range flight or are overspun at the muzzle. Sometimes this is referred to as "overstabilized", which is a misnomer, because as this condition continues the yaw will become so exaggerated that the bullet will become unstabile and tumble. What it is doing is spinning too fast. It also is why only properly designed long range bullets will decelerate back through transonic in a stabile and predictable manner. The collapsing shock waves of transonic phenomena will destabilize any bullet with exaggerated yaw. Very, very few bullets exhibit constant drag coefficients throughtout their flight. The concept that these BC variances fall into velocity brackets is an oversimplification of the physics involved, and leads to common mistakes in trajectory calculations. Most manufacturers (myself included) publish BCs that accurately describe the bullet's flight at the muzzle. It is the most accurate and predictable method. To accurately predict your bullet's trajectory from your rifle I would need to know not only the true velocity and twist rate, to the tenth of an inch, but the engraving characteristics, # of lands, depth, etc., and the muzzle exit yaw numbers. Or you can shoot your rifle at the various ranges, using the published BC as a reference, and see where the bullet is actually flying. This is the best way to determine what your rifle, with your load, is doing. [ 06-13-2001: Message edited by: Warren Jensen ] [/QUOTE]
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my drop chart doesn`t match at 500yds and beyond?
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