Do Bullets Go To Sleep?

Discussion in 'Technical Articles - Discussion' started by ADMIN, Mar 11, 2013.

  1. ADMIN

    ADMIN Administrator

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    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.
     

  2. aaronraad

    aaronraad Member

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    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? :)
     

  3. Michael Courtney

    Michael Courtney Silver Member

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    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.

    [ame="http://www.youtube.com/watch?v=4pF8W5liSRc"]Pitching and Yawing of a bullet - YouTube[/ame]

    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.

    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.
     
  4. BountyHunter

    BountyHunter Writers Guild

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    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
     
  5. Michael Courtney

    Michael Courtney Silver Member

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    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
     
  6. BountyHunter

    BountyHunter Writers Guild

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    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.
     
  7. ajhardle

    ajhardle Well-Known Member

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    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: Jul 31, 2013
  8. Michael Courtney

    Michael Courtney Silver Member

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    Were the bullets keyholing, did you measure drag with two chronographs, or did you infer instability from group size? In my view, there are too many potentially confounding factors to reliably infer stability from group size, especially with the small sample sizes typically used by hobbyist reloaders.

    Using sample size to indicate stability probable requires both working hard to remove and document all potentially confounding factors and measuring something like 10 five shot groups under each condition and reporting the mean group size and the standard deviation in the group size under each condition to determine if the difference in group sizes is significantly different (in the sense of a statistically significant difference), as well as reporting all the conditions of each measurement (temperature, humidity, ambient pressure, measured twist rate, bullet dimensions, etc.)

    We've noticed that bullets from some lots are dimensionally different from other lots and can also have lengths and weights that vary from the manufacturer's published specs. Barrel twist rates can also vary from manufacturer specs and should be confirmed to accurately infer stability. Likewise, ambient conditions (temperature, humidity, barometric pressure) should be measured at the site with something like a Kestrel. Still, there are other factors like barrel fouling that can be hard to quantify or repeat exactly from one day to another.
     
  9. ajhardle

    ajhardle Well-Known Member

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    No keyholes, and I don't have the means to measure drag. I'm not and educated ballistitician, but I plan to run test, shooting 5 shoot groups at multiple targets placed out to 300 yards, targets lined up so they are shot simultaneously, record each shot on every target, and repeat the test as temperature changes and at different elevations.

    I intend to show that as air density increases, group size of a stability "compromised" projectile can increase at short range with little affect at long range. It's not the most scientific, but it may be all the effort I am willing to put forth when the solution would be as simple as choosing a proper twist/bullet combination.

    Now if I can duplicate the results?? I'll let you know. This may take a while.

    Sorry about the tangent, but as for the matter at hand, why did you choose a bullet so short in length? Why not a secant BT?
     
  10. Michael Courtney

    Michael Courtney Silver Member

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    You have a nice experimental design. We've often considered shooting through multiple targets to assess dispersion at different ranges, but never tried it. Too much concern for human error (our part) and wind drift. Thought we might have to do it at night to have sufficiently still conditions, but then many ranges do not allow shooting at night. We have figured out that very thin transparencies are probably the best intermediate targets. It is easier to line up transparencies than paper targets, especially if a laser is available.

    We chose the 40 grain bullet for several reasons. 1) We needed a bullet we could slow down to Mach 1.2 and still be stable in the 1 in 12" test rifle. 2) We wanted a bullet with good accuracy in the test rifle across a range of muzzle velocities. Prior work with this bullet demonstrated this. 3) We wanted a bullet that had excellent shot to shot consistency with drag/BC determinations. A lot of other bullets have significantly larger shot to shot variations in drag/BC than this one. The differences in drag we observed were not too much smaller than the uncertainties in drag. A bullet with larger shot to shot variations in drag would mask the effect we were hoping to observe.
     
  11. Michael Courtney

    Michael Courtney Silver Member

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    Personally, I think most long range hunters should be measuring the BCs of their chosen hunting loads. We've measured a lot of BCs and also thoroughly reviewed the results of all the BCs measured by Litz and reported in his book on Applied Ballistics (highly recommended). BCs routinely can be 10-15% different from manufacturer claims (usually lower), can vary with the rifle the bullets are shot from, and are sometimes more than 40% lower than manufacturer claims. Using a BC from manufacturer claims, or even independently measured BCs from Litz or us can lead to unexpected performance at long range.

    Some folks think about BC just in terms of drop and fail to consider that lower than expected BC also means greater wind drift and lower impact velocity.

    Measuring your own BCs is as easy as adding a second chronograph to a standard long range shooting setup. With 0.3% accuracy, the CED Millenium has worked well for us, though we perform an extra calibration step to improve the relative accuracy for near and far velocities to 0.1%. (Probably not needed for most long range shooters). If the chronographs are spaced a carefully measured 100 yards apart, you can accurately determined the BCs of your bullets from your rifle by entering the near and far velocities in the JBM BC calculator along with the environmental conditions.
     
  12. ajhardle

    ajhardle Well-Known Member

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    So, what can the regular old redneck with a gun like me take from your article?

    Well.....I have typically placed a chrono 15ft from the muzzle and used a B. Litz provided B.C. and the provided correction from the ballistic calculator results in a shot hitting low, at extreme ranges, approaching the sonic barrier. If assumed my bullets were yawing, even though I couldn't tell from round holes in the paper.

    What I've learned: if I were shooting a .215 g7 B.C. @2600fps, but it had a significant wobble for the first 100 yards, let's say it averaged 10% more drag, (just a convenient number) it would lose velocitylikebullet with a bullet with a .194 B.C. The difference would be that at 100 yards, the bullet would be traveling 2373 fps, not the predicted 2395. What difference is that? That's 10"at 1000 yards at sea level. If I correct my muzzle velocity to 2577, I can correct for the lost velocity.

    So what does a shooter do with this knowledge?

    I think it's worth while to record velocity beyond 100 yards to use in a ballistic calculation. That's what I will do from now on, unless you have better suggestion....
     
  13. TracySes23

    TracySes23 Well-Known Member

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    I've never used a chronograph, but around 69 thru 71, my best 5 shot group was 5/16" at 100 yards with a Rem, 788 chambered for the 22-250. Never was able to do better than that, but managed a fair number of 5 shot groups under 1/2" a couple of them were 3/8".
    I decided to try my luck at 200 yards hoping I could hold the same MOA. I was taken aback when my first group was 5/8". I never did better than 5/8" at 200 yards after that, but after going to the range 6-8 more times, I managed to get about 25% of my 5 shot groups just as small. i also managed to get about 45% between 3/4 - 7/8" 5 shot groups. I'm recalling from memory, but I don't think I had more than 2-3 groups go over an inch at 200 yards. The load was 35.5 gr 4064 behind a 55 grain Speer Spitzer. I know that only because, when I dug out all my reloading paraphernalia, the labels on my ammo boxes told me what load I was using.
    My experience wasn't very scientific, but the MOA of the groups were so consistently better, using the same load, I could never come up with a better explanation. I have to believe something is responsible for those better groups & more consistent groups.
    Unfortunately I was laid off the same year and had to move to Illinos to find a job. Never found a range close enough until recently to shoot from a bench. Two more years after that, I got divorced & was too poor to buy reloading supplies for a long time. As it is now I drive 100 miles round trip each time I want to shoot.
    To top it all off, I live in Illinois, which really sucks even with the new "Concealed Carry" law that's supposed to be enacted this coming January.

    If anyone has an explanation better than mine, I'd like to hear about it.
     
  14. Michael Courtney

    Michael Courtney Silver Member

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    Anecdotal observations lile these are tantalizingly interesting.

    Purusing some recent benchrest match group sizes at 100 and 200 yards shows that about 80% of the groups grow in average size (in MOA) between 100 and 200 yards, and about 20% of the groups shrink in average size. Only 10% of the groups shrink in size by an amount that can be considered statistically significant.

    When only 10% of the available data shows the shrinking group sizes with longer range, the question becomes whether this observation can be dismissed as a result of random chance, or whether it is systematically produced by some feature of the rifle barrel, bullet, and conditions. Anecdotally, we have noticed that the feature of shrinking group size seems to occur commonly for combinations that are only weakly stabilized, such as a 53-55 grain .224 bullet in a 1 in 14" twist.

    Unfortunately, the available benchrest data on group sizes does not contain enough detail to analyze the data with some meaningful selection criteria that would allow assignment of a possible causal factor to the shrinking groups at longer range. To compute stabilities, we would need a record of bullet make, model, and weight, barrel twist rate, muzzle velocity, ambient temperature, humidity, and atmospheric pressure.

    Consequently, we cannot yet determine whether or not the occasional occurance of smaller group sizes at longer range is a meaningful and predictable consequence of definite factors or whether it is the result of random chance.