Sierra 6mm 107gr Matchking BC

Discussion in 'ELR, Ballistics & Bullets Board' started by SteveOak, May 13, 2018.

  1. SteveOak

    SteveOak Silver $$ Contributor

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    The Ballistics Calculator on the Applied Ballistics website lists the G1 BC for the Sierra 6mm 107gr Matchking as .508 and the G7 BC as .261.

    Are those numbers for the 'new' factory pointed version or for the 'old' non-factory pointed version?
     
  2. #40Fan

    #40Fan

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  3. SteveOak

    SteveOak Silver $$ Contributor

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    You may be right. I found what I believe to be an old reference to a Sierra published BC of .527.

    That would be a 3.79% increase in BC which is within the range of improvement I have seen for other bullets.
     
  4. shoot4fun

    shoot4fun Gold $$ Contributor

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    Dia. (inches) Weight (grains) Sectional Density Ballistic Coefficients and Velocity Ranges
    0.243 107 .259 .547 @ 2500 fps and above
    .542 between 1800 and 2500 fps
    .529 between 1600 and 1800 fps
    .519 @ 1600 fps and below

    I admit to being long ago confused by the way Sierra lists BC. It seems they list them on their website with G1 (.547) but show the BC dropping with velocity. I never know if you chose the BC for input into ballistic programs according to muzzle velocity or does their "chart" mean the BC actual drops as the bullets slows down?
    o_O
     
  5. swd

    swd Gold $$ Contributor

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    .547 is real close using the Berger ballistics program.
     
  6. SteveOak

    SteveOak Silver $$ Contributor

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    Sorry, I don't follow. The Berger ballistics program I am familiar with, http://www.bergerbullets.com/ballistics/, lists only Berger bullets.

    Or are you plugging .547 in to the Berger program and then comparing it to something?
     
  7. shoot4fun

    shoot4fun Gold $$ Contributor

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    JBM, if I read it correctly, uses the stated Sierra BCs (G1) in its calculation. I also haven't seen anything but Berger bullets in their calculator.
     
  8. #40Fan

    #40Fan

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    I could never figure it out either, but I don't know if I have ever shot one of their bullets slower than the top speed/BC listed, so I always used the highest number.
     
  9. swd

    swd Gold $$ Contributor

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    Yep, just plug in .547 and change any other appliacable paramaters such as temp and altitude, etc.
     
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  10. South Prairie jim

    South Prairie jim

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    This is a timely thread gents as I've been using.527 not that it makes that much difference for benchrest shooting.
    My question for this thread is where is the lot numbers for Sierras 107 I'm confused
     
  11. Laurie

    Laurie

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    It's not just the speed the bullet starts out at from the muzzle. The point about banded speeds is to get the BC over the whole flight as the bullet slows. So, in a 1,000 yard distance shot all of the different BCs come into play typically. Sierra's own Infinity ballistics program automatically changes the BC over distance / dropping residual speeds. For a long-range shot using other calculators, averaging Sierra's various BCs is usually reasonably close, albeit through a very unscientific way of doing things. :)

    Here is what Bryan's Litz's field tests produced for the two versions of the 107gn SMK as average G1/G7 BCs:

    original unpointed .................. 0.507 G1 / 0.260 G7
    new pointed version ............... 0.499 G1 / 0.256 G7

    Yes, I've typed that correctly - the pointed model gave a slightly poorer result in his tests. o_O

    This is one of the very few instances where the newer pointed variant shows a deterioration for whatever reason. (Source: Ballistic Performance of Rifle Bullets 3rd edition.)
     
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  12. goodi

    goodi

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    Reply from Rich Machholz when the new sierra pointed 6mm 107's were released

    The G1 BC for the pointed 6mm 107 is .563.
     
  13. ASbobcat

    ASbobcat Silver $$ Contributor

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    There is one possible explanation for the reduction.
    There are two commonly available pointing systems, and it appears from the shape of the ogive that Sierra chose to use what may be the "less optimum" version. I have used both, and found that one produces a more uniform ogive than the other.
    If you look closely at the tip of the "new" 107's, you will see an angle in the ogive. That angle produces an oblique shock that induces drag. It may be significant near Mach 3. I found an old NACA report ( #1673) from 1948 that shows the exact numbers. The report is available for download on https:/www.digital.library.unt.edu. Look on page 11 at M2.8 (approx 3100fps) and you can see that a change in ogive angle exceeding 6deg produces significant aerodynamic losses.
    I run the new Sierra's through the "other" style die, which reduces the angle significantly.

    I intended to do a 250yd "relative drop" test yesterday, but range trip cancelled. As soon as I can get to the range I will report on actual drop numbers for pre and post "re-pointed" versions to see if it makes a difference.
     
    Last edited: May 19, 2018 at 11:07 AM
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  14. SteveOak

    SteveOak Silver $$ Contributor

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    Comparing pointed Sierra 6mm 107gr MKs to the old not pointed version?

    The article sounds really interesting. Unfortunately that link took me only to the home page. Searching for #1673 got me 7,784 returns. Could you post the title of the paper please?

    Perhaps it is good that I am launching the Sierra 107s at 2,850. :)

    I am very interested in this thread for a couple of reasons. I enjoy exploring the physics of bullet flight and if I can find a bullet what is appreciably better (1" at 10 MPH 90 degrees in a 6BR) in the category of wind deflection at 385 meters I would be prompted to switch.
     
  15. ASbobcat

    ASbobcat Silver $$ Contributor

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    The title is "Tables and Charts of Flow Parameters Across Oblique Shocks" You could search by that or "NACA Technical Note #1673".

    I can tell you that a velocity difference of a couple of hundred fps (M approx 0.2) is not nearly as important as the angle of flow change.
     
  16. ASbobcat

    ASbobcat Silver $$ Contributor

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    Here are some diagrams/photos to help visualize an oblique shock. The diagram titled "Isentropic Flow Expnasion" shows flow around a smooth (no angle) surface. "Isentropic" in layman's terms, means no aerodynamic losses (no change in Entropy). For comparison, note the dark lines in the Schlieren photo of an X-15 model at high Mach number. Observe that shocks coming from curves of the aircraft behind the primary shock from the nose are very weak, but that there is a very strong shock where there is a sharp angle on the shaft holding the model from behind. The darkness of the lines is a direct visualization of the difference in air density across the shock, inferring inefficiency, or increased drag.. Isentropic Expansion.png Oblique Shocks X-15.jpg
     
    Last edited: May 19, 2018 at 4:37 PM

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