338 300gr BC

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by canderson, Oct 1, 2010.

  1. canderson

    canderson Well-Known Member

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    Hey guys,
    I am having a little difficulty in getting a ballistics program to match the drops I am getting with my 338 Allen. I have tested the velocity with two different chrongraphs (they both measure within 30 fps). The corrections with several different programs will be at least 1 MOA from what I actually have to use to hit the target. Any help is greatly appreciated as barrel life is very precious on this one. Thanks in advance.
    Chris
     
  2. jeffro

    jeffro Well-Known Member

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    Have you checked your scope to see if the adjustments are tracking correctly.
     

  3. canderson

    canderson Well-Known Member

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    Yes, it has been awhile. It is a nightforce and I believe the clicks were just a little over the 0.25 moa that is stated (0.258).
     
  4. jeffro

    jeffro Well-Known Member

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    What 300gr are you using, and what program.
     
  5. trebark

    trebark Well-Known Member

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    I have found with most software that the 'ideal' trajectory rarely matches reality. I use exbal and it has a trajectory validation function that helps to move 'ideal' to 'reality'
     
  6. jmden

    jmden Well-Known Member

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    yep...yep...doing this should get you squared away quickly regardless of what your chrono says. Don't get too hung up on what the chrono says...it's much more important, obviously, to hit what you're shooting at by matching the program to your actual trajectory. Might try using Bryan Litz's bc number (g1 or g7) for the 300g pill you're using. I'm shooting the 300SMK at 2980 and using his numbers for it, I haven't even had to do any trajectory validation, but I usually do with other loads/rifles. Good luck.
     
  7. jwp475

    jwp475 Well-Known Member

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    30 FPS off will cause a lot of problems. Use the trajectory validation feature on your ballistics program
     
  8. canderson

    canderson Well-Known Member

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    Thanks for the input. I will look into the trajectory validation. The puzzling part is that with my f class rifle, the trajectories are usually within 0.25 moa out to 1000.
     
  9. Down Under Hunter

    Down Under Hunter Well-Known Member

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    I have a feeling that Kirby was running a custom BC for his AM as well. Might have been .81 in his gun ? Not sure. I use the .768 in exbal and 2832 MV in my edge. This is dead on for my gun .The chrono gives it 2870 at the muzzle and 2852 fifteen meters down range.

    I wouldnt worry what the chrono says either..

    DUH
     
  10. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    I actually use .820 for BC with my 338 AM and .800 for my 338 AX. Have always worked out fine for me.

    The difference in trajectory at 1000 yards with 30 fps velocity spread is roughly 1/4 moa. Far less them most of us could tell with our shooting ability, putting me on the top of that list.

    One thing you may have to play with is multipul BC values for various velocity ranges. THis can also help you tweak your drops.

    Are your drops consistantly off over the entire trajectory or more one ended?
     
  11. trebark

    trebark Well-Known Member

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    With an ES of 30+ you will have a hard time matching a calculated long range trajectory.
     
  12. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Trebark,

    How do you figure that?

    Check it with one velocity and see if it matches up, if not check the 30 fps faster velocity.

    In most cases, 30 fps is not worth even worrying about, average the difference and use that velocity for your program.

    30 fps spread will not make 1 moa of difference at long range(1000 yards) Not even close.

    Here is an example of 30 fps variation at 1000 yards: My 338 AM with 300 gr SMK

    3300 fps velocity and data at 1000 yards:
    Retained velocity: 2303 fps
    Retained energy: 3532 ft/lbs
    bullet drop w/ 100 yard zero: 16.75 moa (174.2")
    bullet drift w/ 10 mph crosswind: 3.0 moa (31.8")

    3330 fps velocity and data at 1000 yards:
    Retained velocity: 2327 fps (only 24 fps difference)
    Retained energy: 3605 ft/lbs (73 ft/lbs difference)
    bullet drop: 16.25 moa (1/2 moa difference)
    bullet drift: 3.0 moa (no difference)

    Now if you can consistantly hold less then 1/2 moa accuracy you MAY be able to see the difference of 30 fps at 1000 yards with this class of chambering but I know very few that can hold this level of accuracy consistantly shooting from field conditions.

    A shooters ability to accurately judge shooting conditions will make MUCH more difference in the real world that 30 fps velocity spread.

    Maybe I misunderstood your comments.
     
  13. Booney

    Booney Well-Known Member

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    What type of speed and group size are you getting.
     
  14. canderson

    canderson Well-Known Member

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    Everyone thanks for helping. Kirby here is a little insight to what I have found. MV is 3200. The rifle is zeroed at 300. 600 drop come up is supposed to be 6.0 moa actual is 6.5 moa. 815yd 11.3 moa (this one was really close). 1000 yd come up 16.5 moa actual is 17.25 moa. 1200 yd come up is 22.8 moa actual was 24 moa. If yall can make sense of what is happening please help. Otherwise, I will back the target out 100yds at a time and see what happens. By the way Kirby, this rifle is a total balst to shoot. I was extremely fun listening to my 2' x 2' steel rings at 1200 today.