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Do Bullets Go To Sleep?

 
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  #1  
Old 03-11-2013, 01:14 PM
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Do Bullets Go To Sleep?

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A bullet can leave the barrel with a significant yaw angle (or tip off rate leading to pitch and yaw) and then pitch and yaw in an oscillatory manner as the peak pitch and yaw angles slowly decrease as the bullet flies downrange. This paper presents an experimental design for detecting the in-flight damping and test results which support the theory of damping of pitch and yaw. Three chronographs were employed simultaneously to determine drag coefficients of bullets over near and far intervals 50 yards long for bullets fired at Mach 1.4 to Mach 3.1. Read More...
This is a thread for discussion of the article, Do Bullets Go To Sleep?, By Michael W. Courtney, Elya R. Courtney and Amy C. Courtney. Here you can ask questions or make comments about the article.
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  #2  
Old 03-13-2013, 12:01 AM
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Re: Do Bullets Go To Sleep?

Straight up, what a fantasitic effort by the authors and researchers, especially considering the equipment employed.

First question is with regard to the calibration process. I didn't identify at which distance the calibration was done. If calibration was done at all three distances (10, 160 & 320 feet), was the difference in readings between the three chronographs still within error limits? I say this, given the tendancy of optical chronographs to be light sensative; and the light might have changed with distance(position), let alone time of day and cloud conditions.

Do you believe the spin, pitch & yaw paths to be truely circular or elipitcal as found more often in nature? If the path is more eliptical and prone to changing with velocity and distance might this impact the measured drag coeffcient each time. I'm assuming that the pitch and yaw act in combination to increase drag, but the combination would have a different effect if the pitch was at the major axis of the elipse and the yaw was only at the minor axis and vice-versa.

Finally, were any chronographs harmed during the making of this report?
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Old 03-13-2013, 07:45 AM
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Re: Do Bullets Go To Sleep?

Quote:
Straight up, what a fantasitic effort by the authors and researchers, especially considering the equipment employed.

First question is with regard to the calibration process. I didn't identify at which distance the calibration was done. If calibration was done at all three distances (10, 160 & 320 feet), was the difference in readings between the three chronographs still within error limits? I say this, given the tendancy of optical chronographs to be light sensative; and the light might have changed with distance(position), let alone time of day and cloud conditions.
The calibration was done with the three chronographs at 10, 12, and 14 feet from the muzzle. The LED skyscreens greatly reduce the influence of changing light conditions. We've calibrated the chronographs many different times (different days, different light conditions, etc.) and we've never had them fail to meet the 0.3% specification with the LED skyscreens, and the accuracy outside is not distinctly worse than the accuracy inside.

Quote:
Do you believe the spin, pitch & yaw paths to be truely circular or elipitcal as found more often in nature? If the path is more eliptical and prone to changing with velocity and distance might this impact the measured drag coeffcient each time. I'm assuming that the pitch and yaw act in combination to increase drag, but the combination would have a different effect if the pitch was at the major axis of the elipse and the yaw was only at the minor axis and vice-versa.

The shape of the pitch and yaw is best described in the above video which is referenced in the paper. The video describes it much better than simple verbal descriptions like circular or elliptical. The theory really is very well worked out by Bryan Litz (see also Epicyclic Swerve ) based on the earlier work of Braun and McCoy. The only thing our new experimental technique reveals is how big the effect is for a specific bullet and rifle and how quickly the pitch and yaw are damped in flight for a specific bullet and rifle. The theory can describe the subsequent motion for a given set of initial conditions, but the magnitude of the effect for a given rifle and bullet depend on the initial conditions.

It is not completely clear if the shot-to-shot variations in drag are due to shot-to-shot variations in tip off rate, manufacturing variations in different bullets, or other experimental contributions to the uncertainty. We've managed to achieve Cd measurements with accuracy in the 1-2% range over the 100 yard interval and in the 3-4% range over the two 50 yard intervals with less than $1k of equipment. $100k of equipment could reduce the experimental error, but it still would not tell you whether you were seeing shot-to-shot variations in the tip off rate or in the Cd of different bullets. We did intentionally pick a bullet where we had observed small shot-to-shot drag variations in the past.

Quote:
Finally, were any chronographs harmed during the making of this report?
You may be remembering the stability papers co-authored with Don Miller where I shot the downrange chronograph. Since that event (Summer 2011), we've been much more careful. Our main upgrade in chronograph safety is having Elya or Amy behind the trigger, while I've been demoted to data recorder when we take data. Elya was the shooter for this experiment, so the chronographs were safe.
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Old 03-13-2013, 08:53 AM
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Re: Do Bullets Go To Sleep?

Maybe I missed the intent, but for years the alleged issue has been the supposed ability for a bullet to go to sleep and shoot much tighter at distances such as 300 yds vs 100 yds. Not one out of ten times but rather 9 out of 10 times. Never seen anyone prove a consistent sleep with tighter LR versus SR groups other than internet claims of inconsistent data at best.

So did that resolve that the issue?

BH
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  #5  
Old 03-13-2013, 09:19 AM
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Re: Do Bullets Go To Sleep?

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Originally Posted by BountyHunter View Post
Maybe I missed the intent, but for years the alleged issue has been the supposed ability for a bullet to go to sleep and shoot much tighter at distances such as 300 yds vs 100 yds. Not one out of ten times but rather 9 out of 10 times. Never seen anyone prove a consistent sleep with tighter LR versus SR groups other than internet claims of inconsistent data at best.

So did that resolve that the issue?

BH
This is a great question that shines light on two possible meanings of bullets going to sleep. If "going to sleep" means damping of pitch and yaw, then yes, bullets do go to sleep. If "going to sleep" means smaller angular groups at longer range because of this damping, then it probably does not happen. See the Litz article: Epicyclic Swerve

However, saying that it probably does not happen because it's been looked for carefully under various conditions without finding it does not prove that it NEVER happens. We need to acknowledge that there might be some combination of rifle and bullet and load out there somewhere that really does demonstrate reduced angular dispersion at longer ranges. It is just that the effect has not been carefully documented in a manner sufficiently convincing that it could be attributed to damping of pitch and yaw rather than some other confounding factor.

In summary:

1. Bullets can have considerable pitch and yaw when they leave the barrel
2. This pitch and yaw increases drag
3. The pitch and yaw are damped out in flight and the drag is thus decreased
4. The effects on trajectory are too small to cause a reduction in angular dispersion with range
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  #6  
Old 03-20-2013, 08:11 AM
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Re: Do Bullets Go To Sleep?

If I remember right, the old 303 #4 MK1 had a well known ability to regulate groups at 900 that were tighter than at 600. However, most assumptions were the harmonics and bedding that caused that, not the bullet "sleeping".

Be interesting to see where this goes.
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  #7  
Old 07-31-2013, 10:40 PM
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Re: Do Bullets Go To Sleep?

First off, thanks for the article. I frequently need a push to get the gears in my head moving.

Now BountyHunter, I'm sure I don't have to tell, but smaller angular groups at longer range is most likely an issue of gyroscopic stability that may exaggerate the pitch and yawing motions, or maybe orbital gyrations? I'm not a scientist (professionally) but it seems to fit. It would be interesting to perform this test with my 75gr Amax load in a 9 twist at 2650 fps. I would think the motions would have a greater affect on drag with the much longer bullet.

As for the tighter groups at longer ranges, I will setup multiple targets out to 300yards, and shoot them all simultaneously. But, that will have to wait, because right now, they shoot great at 100 yards. I guess 90 degrees at 4300 ft is enough to properly stabilize them. When I started developing the load in 30 degree temperature, it was not. To me, that was enough evidence to confirm, but I might be fooling myself?

Last edited by ajhardle; 07-31-2013 at 10:49 PM. Reason: left out half a sentence.
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