Is my BC off or my Chrono?

Discussion in '6PPC, 6-6.5x47, 6XC, 6 Rem, 243' started by Timgunner, Feb 14, 2018.

  1. Timgunner

    Timgunner Silver $$ Contributor

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    I’ve had a strange occurrence lately that I cannot explain. I bought a rifle from an estate sale. It was a custom 6.5x47 lapua and it came with loaded ammo. The loads stated 120gr amax with re15. I sighted the rifle in and the loads shot great. I didn’t have my chronograph that day so I guessed the velocity at 2900 and plugged it into my Applied Ballistics phone app. To my surprise my drops were dead on! I took the rifle hunting and killed 6 deer from 200 all the way to 980 with the load and ballistic data. Fast forward a couple months. I’m running out of loads from the previous owner so I load up some identical loads, assuming his data was correctly annotated. I take those new loads and the old ones to shoot side by side. To see if I can replicate the accuracy I had previously seen. To my surprise the Oehler 35 read an average velocity of only 2745fps. What is going on? How are my drops on with a velocity of 2900 but my bullet is only going 2745?
     
  2. nonnieselman

    nonnieselman

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    Did you pull any of the other loaded rounds to see if the charge weight was the same as what was recorded?
     
  3. Timgunner

    Timgunner Silver $$ Contributor

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    No I didn’t pull any loaded rounds, but the velocity data was from his loads not mine.
     
  4. barefooter56

    barefooter56

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    These articles may help:
    http://www.appliedballisticsllc.com/Articles/BallisticCalibration.pdf
    http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf
     
  5. chkunz

    chkunz Gold $$ Contributor

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    We have no way to calibrate our chronographs. They are probably good for complaining velocity differences but have their limitations for the measurement of absolute velocity and even more uncertainty when comparing data from two instruments. The manufacturer will quote an uncertainty in +/- % and it is good to keep that in mind. When comparing two instruments the uncertainty is larger than for either of the instruments alone.
     
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  6. Zero333

    Zero333 Gold $$ Contributor

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    Out to 500 yards you'll still hit a deer's vitals, no problem. Even at 600 yards the drop would be about a 10" difference... but at 950 yards I can't see how you hit the target with 150 fps less muzzle velocity.
     
  7. ttfreestyle

    ttfreestyle Gold $$ Contributor

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    Confirm your 100 yard zero.
    Go to 1000 and true your load with your app, targets don’t lie. Chronos do at times in my experience.
     
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  8. mr.big

    mr.big

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    yeah there is no way to know what you were shooting,,,I have ammo boxes with loads written on them that the actual ammo in them isnt even the same caliber ,much less the same load,,even if it was a lot of RL15 from when he loaded could be 150 fps faster than what you loaded with your lot of RL15,,
     
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  9. Timgunner

    Timgunner Silver $$ Contributor

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    The only thing I can think of that makes any sense is that those were loaded with 140 amax instead of 120 amax. After running the numbers, by the time they make it to 980yds, a 120gr at 2900 is only .7 moa difference in drop than the 140gr at 2745.
     
  10. XTR

    XTR

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    pull one down, weigh everything.
     
  11. boltfluter

    boltfluter

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    RL15 is a great powder in general. But, it is very temp sensitive if you are shooting it year round. It will drive you crazy with the ever changing velocity. I changed to Varget for this very reason and everything fell into place. Just my personal experience. Good luck and good shooting.:D:eek::D

    Paul
     
  12. WyleWD

    WyleWD Gold $$ Contributor

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    Did you ever run any of his loads over your chrony, then run run yours over the chrony? I'd agree with the others that have said to tear one of his loaded rounds down and confirm bullet, powder, and primer too. JMHO. WD
     

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