Re: Load development yardage testing?
It is known as gyroscopic precession. This is the motion that a bullet has when it is in flight. Sometimes this effect is minimal and sometimes it is so bad as to have a negative effect on the flight of the bullet. It is likened to a quarterbacks pass. Sometimes it flies perfectly along its trajectory with no coning motion. Other times it has a pronounced wobble that bleeds off energy and range faster. This has many causes. One of the common causes in military ammunition is the core of the bullet not being precisely located in the center of the bullet. This effect does stabilize with range. It is a common reason that some rounds will group poorly at close range and get better as the range increases.
There is a little bit of an explaination on this website also.
http://www.frfrogspad.com/extbal.htm
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> We mentioned that CD can be estimated fairly well from certain bullet dimensions. However, because of the effects of bullet wobble (precession due to rotation), nose tip radius or flatness, nose curvature and boat tail, boundary layer interaction from cannelures and land engraving, etc. (all of which affect the wave drag, base drag and friction drag of the bullet differently) it is really impossible to predict with total accuracy the actual CD vs. Mach number. Also, while a ballistic coefficient can be computed from velocity measurements at two points, differences in bullet wobble diminishes the validity of chronograph testing for BC change over separate series of different muzzle velocities--it needs to be done by separate measurements at different ranges for each shot. Why? Read on.
An elongated bullet, as opposed to a round ball, is inherently unstable aerodynamically. When made stable gyroscopically by spinning, its center-of-gravity will follow the flight path. However, the nose of the bullet stays above the flight path ever so little just because the bullet has a finite length and generates some lift. This causes the bullet to fly at a very small angle of attack with respect to the flight path. The angle of attack produces a small upward cross flow over the nose that results in a small lift force. The lift force normally would cause the nose to rise and the bullet to tumble as the nose rose even more. That is where the spin comes in and causes the rising nose to precess about the bullet axis. When the spin is close to being right for the bullet's length, the precessing is minimized and the bullet "goes to sleep" If it is too slow the bullet will not be as stable as it should. (That is why Jeff Cooper says it's wrong to shoot groups at 100 yards for accuracy testing and suggests 300 yards. If your twist isn't right for the bullet used your group size will be larger at long ranges than would be expected by extrapolation of 100 yard data due to bullet wobble.)
Of course, any other disturbing force such as a side wind gust could cause a difference in bullet nose precession but the effect would be quite small for a properly spin stabilized bullet. Most of the lift force is on the nose of the bullet and is proportional to the square of the bullet velocity as well as the nose shape and length. The new long-nosed bullets for long range match shooting can generate quite a bit more lift occurring farther ahead of the center- of-gravity and can produce a nasty pitch-up moment. That is why they require a faster than normal twist to stabilize them. Pistol bullets, being relatively short and with little taper to the nose, require a slower spin for stability.
Let's look at the rotational speed of a bullet. The formula for computing the rotational speed of a bullet is
R = (12/T) * V
where
T = Twist
V = Velocity in f/s
R = Rotations per second
Now consider a bullet chronographed at about 2750 f/s muzzle velocity fired from a rifle with a 10" twist. It is rotating at around 198,000 rpm Let the flight velocity decay to 2000 f/s. Now what is the bullet rotational speed? Well, it doesn't fall off much because the only things slowing it down are inertia and skin friction drag which is pretty low, and with the M80 ball bullet it has been measured about 90 percent of the original rpm (or in this case about 178,00 rpm) depending on the bullet. Then chronograph an identical bullet from the same rifle, this time with a muzzle velocity of 2000 f/s. Its rotational velocity will be 144,000 rpm. Its stability will be different from the bullet fired at 3000 f/s and allowed to slow down to 2000 f/s. It will not have the same drag at 2000 f/s although the bullets are identical. Therefore, two identical bullets fired from the same rifle at different velocities, will not have the same drag coefficient or ballistic coefficient just because of the way the measurements were taken. There are times when test data does not mean what you think it does. Again, radar range testing is the only way to fly for trustworthy bullet drag data. [I am indebted to Lew Kenner for this lucid description of bullet stability.]
Another factor is that it is not necessarily true that the drag coefficient of a particular bullet is proportional to that of another bullet of the same design across the Mach number range, but this is what a ballistic coefficient assumes.
Something else to worry about is the effect of the bullet tip shape/condition on the ballistic coefficient. Because modern bullet have soft points they are subject to damage and manufacturing tolerances that can alter the BC from bullet to bullet and across otherwise similar bullets, although this affect is small unless there is a great deal of deformation.
For truly accurate results, individual bullet characteristics need to be measured on Doppler radar ranges as is done by the military--much too expensive a procedure for the commercial bullet industry who doesn't really care about great accuracy in BC calculations--and the drag model from those measurements applied only to the particular bullet tested. (If you have a spare $100,000 + and would like to buy me such a setup, let me know.)
The good news is that for normal rifle ranges the drag coefficients and ballistic coefficients can work satisfactorily for most purposes--so let's proceed.
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