Pressure flats

Discussion in 'ELR, Ballistics & Bullets Board' started by tomswede, Jan 8, 2018.

  1. tom

    tom Gold $$ Contributor

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    I'm sure when I actually tune it 1k, I'll look back and confirm I have no clue what 300 even means lol.

    Tom
     
  2. rhovee

    rhovee

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    I'm really curious what you end up with. Or if you ran that charge out to 1k how it would do. Thinking about trying this test out in my hunting rifle.
     
  3. tom

    tom Gold $$ Contributor

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    I've put 40,000 rounds on paper at 1,000 yards since 2008.... Odds are favoring I WILL lol.

    Just need it to break out of winter a bit more.

    Tom
     
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  4. Bart B.

    Bart B.

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    I've found Sierra Bulllets' load data helpful in calculating the fps velocity change per 1/10th grain change in powder charge. Their velocity increments are 100 fps. Divide a 100 fps velocity change charge weight change by the number of tenth grain powder increments.

    If a 1.5 grain in charge weight changes a velocity 100 fps, 100 divided by 15 equals 6.7 fps change per tenth grain change in charge.

    For example, here is Sierra's 308 Win load data; search for this phrase then use the CityMaker 6mmbr link to download the file

    "sierra 308 win.pdf"

    Calculate a bullet-powder data change for each 100 fps band then note the fps per tenth grain change for each band.
     
    Last edited: Jan 21, 2018
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  5. retired

    retired

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    i think you need to call sierra about your math and their numbers.
    i doubt any 2 loads came out in 100fps steps.

     
  6. Bart B.

    Bart B.

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    Here's Sierra's 100 fps velocity steps for 308 Win with 175 and 180 grain bullets:
    Screenshot_20180121-140835_crop_315x488.jpg

    Not good enough for you? It's high school math and curve plotting. It shows trends and approximation. Same as all load data based on anyone's objectives, standards and conditions.

    If not, then you tell Sierra about it. It's their data. I'm just the messenger. I shoot back.
     
    Last edited: Jan 21, 2018
  7. retired

    retired

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    It shows trends and approximation.


     
  8. dkhunt14

    dkhunt14

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    That is just a rough guideline. Different barrels, even from the same company can really change the velocity. I've had fast barrels and slow barrels. Also different lengths can really change it also. Usually a longer barrel is faster. I have also had slow lots of powder and fast ones and the velocities really change . Matt
     
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  9. Bart B.

    Bart B.

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    Of course, it's a guideline. That data I put up showed what one set of components produced.

    There will be different velocity numbers across different ways the rifle is held. Several people will have different velocities with the same rifle and ammo; I've seen over 50 fps spread in average. Each lot of powder and primers will have their unique set of velocity numbers over a small range.
     
    Last edited: Jan 24, 2018
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  10. Ned Ludd

    Ned Ludd Silver $$ Contributor

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    Which is exactly why it makes more sense for the manufacturer to simply list the G7 BC values, which are relatively insensitive to velocity. Then the thousands of end users don't each have to do the same thing on their own, or simply live without the best information when they're trying to do load development or compete. Advertising high velocity band G1 BCs is, in part, exactly that...advertising. It allows the manufacturer to list a slightly higher BC value, which may have a positive impact on sales for anyone comparing their product to a competitor's product. It may not be the sole reason, but it's at least a part of it. As long as the provided information is factually correct, there's nothing necessarily wrong with this approach, and in fact, advertising has been carried out this way since its inception. However, providing consumers with the most useful product information, even if it may not allow a manufacturer to distinguish their product as well from that of a competitor, can also generate consumer good will and confidence, which might also lead to increased sales.

    As far as the pressure/velocity flats, they do exist. If you aren't seeing them, the easiest thing to try is test a few different primers. I have no idea whether loading to the center of pressure flats is a commonly used approach in other types of shooting, but there are plenty of F-Class shooters that pay close attention to them. If you load to the center of a charge weight window where pressure/velocity increase minimally relative to other regions of the charge weight/pressure/velocity curve, you are also loading to a region where velocity is likely to be minimally affected by temperature changes. That can make a difference over the long (20+) shot strings we typically fire in F-Class matches.
     
  11. Cowtownup

    Cowtownup Silver $$ Contributor

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    Tom, did you shoot this ladder horizontally to look for POI shift while you got velocity data?

     
  12. tom

    tom Gold $$ Contributor

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    Yes, just moved one dot each increment and looked for pressure signs.


    Tom
     

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