Bullets and BC's... thought experiment

Discussion in 'Practical Precision--PRS, NRL, ELR' started by SAPenguin, Jun 10, 2019.

  1. DocUSMCRetired

    DocUSMCRetired

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    We actually do know what the spin rate is at the target, devices are out there to measure this. We have done these kinds of tests in our lab.
     
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  2. 284winner

    284winner Gold $$ Contributor

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    I think you do have answers. What you stated above was pretty much my experience shooting heavy vs light (same BC) at beyond 1200 yards. Tough to deny experience in the field vs theory ( ballistic programs) when Ma nature comes to play. We're not seeing the 338/375s in the ELR Match's because the 195s from the big 7mm cartridges shoot as well with the same BCs at even high velocities. There's a reason for it. I don't have answers for this other than proof is at these ELR events.
     
  3. Longrange57

    Longrange57

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    Doc, is the info available and does it coincide with Kolbeys formula?
     
  4. Longrange57

    Longrange57

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    284, I follow you. Guys here have great points. I was not aware of devices for purchase to measure spin at distance. Was not aware that this testing was done as Doc indicated. Hope it is info we can obtain and make observations from. I'm trying to help a friend and both of us understand what may be going on at 2200 to 2500 with a .375 and muzzle vel of 2930. Shows to be stable, but occasionally hits would show otherwise. Regular hits at 2500 yds. At 2600-2700 it starts to go subsonic and that may be our whole issue.
    I personally shoot a 6.5 x 284 at 1400 yds (.670 BC) and have a blast! However, a 300 WSM with Berger 210, BC of .621, will shoot inside of me all day long in the wind! LOL
     
    Last edited: Jun 12, 2019
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  5. DocUSMCRetired

    DocUSMCRetired

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    It is state of the art circa decades ago, and requires only materials from home depot on the cheap end of doing it. Just build a spin box, and shoot through it.
     
  6. Longrange57

    Longrange57

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    Sorry to bother you! I see that spin rate decay truly is a non issue....at least to Applied Ballistics!
     
  7. Ballisticboy

    Ballisticboy

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    While knowing the spin rate down range is nice and allows estimates of the spin damping coefficients, it does not help a great deal in assessing the stability of a bullet. The gyroscopic stability will increase as the bullet moves down range unless the bullet is a special with spin control devices. Trying to get a realistic estimate of the dynamic stability will still be extremely difficult and any modelling will be fraught with danger. I have managed to model dynamic instability using a six degree of freedom model but how close it is to reality is debatable.

    We have used a doppler tracking radar to measure the down range spin rates of projectiles.
     
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  8. DocUSMCRetired

    DocUSMCRetired

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    This unfortunately applies to a number of variables, especially when it comes to 6DOf.
     
  9. Ballisticboy

    Ballisticboy

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    This is why they should only ever be used by people who know what they are doing.
     
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  10. JPeelen

    JPeelen

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    Alas, an Internet search for spin box is heavily contaminated by the computer user interface element of the same name.
    Can you point us to a good source describing how a spin box works ? Thanks.
     
  11. damoncali

    damoncali Bullet Maker Site $$ Sponsor

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    For a quick demo of how difficult some of this stuff is, you can try out Lapua's 6DOF phone app. There is a feature where it shows a chart of Sd vs Sg. I plugged in some typical .338 loads, and found that it predicts dynamic instability at unrealistically short ranges. That feature is basically worthless. I have no idea how Lapua gets their areo coefficients, but the 6DOF program isn't terribly useful in the form they provide. It seems like more of a marketing gimmick than an actually useful piece of software.
     
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  12. DocUSMCRetired

    DocUSMCRetired

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    If you have not read it, we have an entire section of lab testing published in "Modern Advancements in Long Range Shooting Vol 1". Where Part 1 is "How Stability Affects Bullet Flight". Chapter 4 of that section specifically is "Spin Rate Decay". https://store.appliedballisticsllc.com/Modern_Advancements_in_Long_Range_Shooting_p/5004.htm. You will find a photo of a spin box on page 60 of this book. You will also find a chapter on bullet stability in our main book. Chapter 10 of Applied Ballistics for Long Range Shooting.
     
  13. Longrange57

    Longrange57

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    Page 169
    Bryan Litz, "It's extremely difficult to calculate how much spin rate is required to overcome a dynamic instability caused by transonic flight. A few of the reasons are: 1. You can't accurately predict the amount of dynamic instability the bullet will have during transonic flight. 2. You can't accurately predict how much the spin rate of the bullet will have decayed by the time it reaches transonic speed."
     
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  14. DocUSMCRetired

    DocUSMCRetired

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    That's why we just measure it.
     
  15. JPeelen

    JPeelen

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    Thank you for the response. I have the book, but had forgotten that the spin box is described in it.
     
  16. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    Although I'm sure there is quite a bit of variability between various bullets/loads, from actually measuring spin rates, are there estimates for the average decrease in spin per distance versus the decrease in velocity? In other words, is there some rough estimate of the ratio at which spin decreases versus linear velocity? My understanding has alway been that the spin rate decreases much more slowly than linear velocity, and thus bullets tend to have a greater Sg as they travel farther, but I've never seen any numerical values ascribed to the spin rate decay.
     
  17. JPeelen

    JPeelen

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    There is 5.56 mm M855 data in a report by SILTON and HOWELL (ARL-TR-5182; dowloadable as ada530985). According to table 9 and assuming normal velocities, after 300 m velocity drops to 68 percent while bullet spin is still at 86 percent of the muzzle value. So the bullet at 300 m does make one turn in about 141 mm of travel, compared to 178 mm at the muzzle.

    Swiss ballistician Beat Kneubuehl for 7.62 NATO reported 74 percent remaining velocity compared to 96 percent remaining spin at 300 m. But he does not indicate how he arrived at this result.
     
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  18. Longrange57

    Longrange57

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  19. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    Yes - I would be happy even just to know something like, "Bullet rotational speed will decrease at approximately one tenth the rate at which its linear velocity will decay"...or something similar to that.
     
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  20. Longrange57

    Longrange57

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    Yes sir, agree
     

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