Chronograph distance and true MV

Discussion in 'Reloading' started by 300 Rem, Sep 23, 2008.

  1. 300 Rem

    300 Rem Member

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    How can one compute the true muzzle velocity (or at least closer to it) when using a chronograph that's not actually at the muzzle? I remember reading about this in the past, but cannot find the article. I routinely set my chrony at 11 feet from the muzzle (closer, and the muzzle blast from my 300 RUM usually knocks off the diffusers).

    My MV's are not up to what's stated in load manuals, and way off what Quickload computes them to be. I understand there's many variables besides the powder type and charge, primer, barrel length, and projectile. The reason I'm wondering about the true MV vs. what I get at 11 feet is because I'm seeing some signs of high pressure. Some cases have ejector marks on the head, but I've never noticed a more difficult bolt lift or any flattening of primers.

    Two loads in question are: .300 RUM = 92.5 gr RL-25, Fed 215, 180 gr. Nosler Accubond (3100 fps) and 180 gr Swift Scirocco (3100 fps); and .300 SAUM = 61.2 gr. IMR 4350, Fed 215, 180 gr. Nosler Accubond (2800 fps). I fellow reloader ran Quickload data on the SAUM load and informed me it was way over pressure. Haven't checked the RUM loads with Quickload.

    Just trying to read and gather as much info as I can before I go and start tweeking what I thought was a satisfactory load at modest MV for the cartridges.

    Thanks
     
  2. Ridgerunner665

    Ridgerunner665 Well-Known Member

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    JBM - Calculations - Trajectory

    Enter your data there...at the bottom of the page it will give your muzzle velocity...or pretty close to it.

    Its usually about 6 fps over the chrono reading on most rifles.
     

  3. Eaglet

    Eaglet Well-Known Member

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    300 Rem,

    When in the field, I use my PDA which runs LoadBase 2.0 and right there I get the corrected muzzle Velocity.

    This two screen shots from my PDA show how the program does it:

    [​IMG]

    On the left it shows the info. required and on the right the corrected Velocity.

    The 1 is 1foot internal distance between the screens.
    The 12 are 12 ft from muzzle to the first screen (other applications do it to the center of the screens)
    The rest of the info is obvious and you can see the correction was 17.2 ft/sec. If I use a BC of 0.631 the correction would have been 6.4 ft/sec as previously mentioned.
     
  4. jmden

    jmden Well-Known Member

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    That's a pretty cool feature of LoadBase 2.0, Eaglet. I wonder how it compares to the method I've used, which I'm sure is not so accurate, but if you're off a couple feet persecond, from what I've seen, it makes essentially no difference. I've messed around with MV in Sierra program or Exbal (little more limited with Exbal in 'reporting distances') and when the speed at the interval of say 20' (or 30' in the case of Exbal as 10 yards is as close to the muzzle as it reports, I think), then just look at the 0 distance line to get your MV. Basically, match your chrono speed at 20' for instance to the 20 ft velocity on the drop chart and then check what the velocity is at 0 ft according to that drop chart. Hope this helps.
     
  5. Eaglet

    Eaglet Well-Known Member

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    jmden,

    I see what you're saying and does make sense. Theoretically it would work, the pain comes in when you start playing with it; ex: 4 yards = 12 ft, set the chronograph at 12ft, if your reading is 3000 ft/sec your chart table would have to read at a distance of 4 yards 3000 ft/sec; then you look at the zero line and what ever it says that should be without doubt your corrected muzzle velocity.
    I tried to go to the motions using different programs and found it kind of painful. Most of us including you would probably hate to be frustrated after a while because you eventually would have to keep changing muzzle velocities at 0 yards until at 4 yards it matches the chronograph velocity. Not to mention that most programs are not willing to give you that info at 4 yards.
    It does work though!!!
     
  6. 300 Rem

    300 Rem Member

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    Thanks for the advice. Eaglet, obviously it makes little difference having a chronograph at 12 feet according to LoadBase 2.0. I appreciate the input.
     
  7. Eaglet

    Eaglet Well-Known Member

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    The difference changes are based greatly on BC, the higher the BC the lesser the change and the lower the BC the higher the change.

    The ballistic application called Ballistic Explorer will correct to Muzzle Velocity using all the conditions like temperature, altitude, barometric pressure, humidity but in that short of a distance what the BC changes does not make much of a difference. I always do use corrected Muzzle Velocity!
     
  8. nfhjr62

    nfhjr62 Well-Known Member

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    The instructions that came with my chrongraph say first screen 10 feet from muzzle and 24 inch between the two screens
     
  9. Eaglet

    Eaglet Well-Known Member

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    It depends what you're shooting, usually the instructions tell you that when using magnum rifles to place them 10 to 12 feet; that's just because of the muzzle blast, when using muzzle brake I ike placing them not less than 12'. Remember, you can place it at 100 yards if you want to, it will still read the velocity at the range; you don't do that if you're trying to find the corrected muzzle velocity but you might want to place one chronograph at 12' and another one at 300 yard if you want to find the BC for the bullet you're shooting.
     
  10. J E Custom

    J E Custom Well-Known Member

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    300 rem

    I still use a 35-p chronograph with 3 screens spaced 2 feet apart.

    Place the first screen 12 feet from the muzzel.

    Fire the first shot and look at the difference between the prof screen and the
    record screen. It will be a little different so if it drops 3 ft/per sec in the 2 feet
    then all you have to do is add 3ft/sec for each 2 foot interval.(3x6=18 ft/sec).

    It works very well if you dont have a computer handy and it's actual muzzel
    velocity no matter what bullet you use.

    I have used a blast screen( a peice of plywood with a small hole to shoot through)
    for the big bores ( 416,458,460 etc ) and placed the chronograph 2 feet from the
    muzzel and then removed the shield and set the screens back to 12 feet and found
    no change in the velocities so I stoped using the blast shield because of setup time.

    So either way (Eaglets,or the way I do it will work).

    J E CUSTOM
     
  11. 300 Rem

    300 Rem Member

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    Thanks, I appreciate the input. That helps