Chrony vs BC

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 284stak, Nov 15, 2010.

  1. 284stak

    284stak Well-Known Member

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    Like to get opinions on which is more believable. Just got back from testing the 160 accubonds in my 7mm. Chronograph by Chrony at 6000ft showed 3125 average velocity. Based on drop from 200 to 800 yds - that would produce a BC of .6, which is substantially higher than the published 0.531. Have others found the BC on that bullet to be in that range ---- or is it more likely that the bullet is actually going more like 3200 and the Chrony is off?
     
  2. elkaholic

    elkaholic Well-Known Member

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    There is definitely no way that a 160 gr. AB has a b.c. of .6! i would say either the chrony (most likely) or a math problem?......Rich
     

  3. Mikecr

    Mikecr Well-Known Member

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    +1 elkaholic
    Also bullet BCs are based on sea level standards(not 6Kft). So having higher local BC at higher elevations(lower air density) is not a surprise really.
     
  4. kemen4

    kemen4 Member

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    Had a similar discussion with a colleague ... having read the Sierra process for determining BC by firing, including the impact of altitude all was set ... until he realised it would be his chrony as the 2nd unit out at the target end of things. Seems the project is on hold and he uses the BC on the box - for better for worse.
     
  5. Kevin Thomas

    Kevin Thomas Well-Known Member

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    A quick comment about Sierra's BC firings; they do indeed use two chronographs, but only three screens. Both chronos are set up with S-1 as being the "start" screen, and the second and third screens (S-2 and S-3) being the "stop" screens for the two chronographs, resepctively. Bill McDonald and I have had many a long chat about this and this is his preferred method of doing it to eliminate errors caused by idiosyncracies of the individual chronos involved. I also used another setup involving two chronos with four screens, which was much faster and easier to set up. Bill eventually signed off on this route, but the BCs have been derived using both methods. I have no idea if they've changed anything in the past few years, but that's how it was done in the past.

    The risks of using two seperate chronographs comes down to the errors inherent in complicating a system; more potential measuring errors, set-up errors, cycle errors in the units themselves, etc.. Best advice I can give here, is measure everything to the best degree possible, and then remeasure to verify it all over again. Distances need to be precise for the numbers to come out accurately (remember GIGO -"Garbage In, Garbage Out"). Also, make sure you know your atmospherics and apply the appropriate corrections. Hope this helps.
     
  6. Idaho Sawyer

    Idaho Sawyer Well-Known Member

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    I have the same thing going on with my 7 and 168 bergers. I went with the chrono speeds and kept adjusting my BC (on my drop charts) to match my actual drop. Turned out to be a BC of .68. I have truthed it out to 1300 yds and everywhere in between. I imagine some barrells smooth a bullet while other barrells rough em up. I always wonder though if my Chrony is spitting out incorrect numbers.