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Thread: Using Chronograph Data

  1. #1
    Member rimfirematt's Avatar
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    Default Using Chronograph Data

    I was the recipient of a Chrony for Christmas!

    I was finally able to take it out for a test drive today. It was pretty neat having it and seeing what my handloads and guns do.

    Now what does the info that I obtained mean? How do I use this data?

    I tried 2 guns today. 1 glock 10mm and a 1911 10mm.

    The Glock with its 4 something inch barrel was getting about 1130 FPS on average. The extreme spread however was anywhere from 40-60 FPS per string.

    The 1911 with its 5 inch barrel was getting about 1180 on average but the extreme spread was 30 something on both strings of 10 shots.

    So near as I can tell your spread is pretty affected by the gun too and not just how carefully your handloading.

    Obviously I need to tinker somemore with this thing to figure it out. But can someone give me a How-to on how to use this Data to improve my Loadings? Or, what will I see from the Data that Im producing quality ammo?

    Also what does standard deviation Mean?

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    You are correct with the firearm making a huge difference in the equation.

    If everything were perfect, each cartridge would have the exact same velocity. But is just doesn't work that way.

    Why do you have higher variation with two sidearms with the same ammo? Barrel length, barrel design (polygonal versus traditional rifling), chamber dimensions/headspace, consistency of firing pin strike it goes on.

    You aren't shooting a Palma match with your heaters, be happy with what you are getting, it looks pretty good to me.

    Standard deviation: best google that, as the answer can get kinda long.
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    Default standard deviation

    If I recall correctly ,standard deviation is equal to the square root of the variance ! When the variance is the difference between the lowest value and the highest value ! so if the difference from the highest to the lowest was 36 the standard deviation would be the square root of 36 or 6. Its been a while but I think that is correct ! Kevin
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    Default well I almost had it ! heres what Wilkapeida says

    Consider a population consisting of the following values
    There are eight data points in total, with a mean (or average) value of 5:
    To calculate the standard deviation, we compute the difference of each data point from the mean, and square the result:
    Next we average these values and take the square root, which gives the standard deviation:
    Therefore, the population above has a standard deviation of 2.
    Little bullets might expand ! But big bullets never shrink !
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    Member rimfirematt's Avatar
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    Whoa! I just lost all intrest in standard deviation for now!

    So is there any standard by wich to judge your handload by how much of an extreme spread you have? As in "gee my spread is only 20fps, that is awesome" what Im trying to get at is if I am producing quality ammo and how to determine that.

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    Applying the standard deviation to your case, ... compute the average of your 10 shots- that is the mean. From the mean, add and subtract 1 sd, 68% of your shots will fall between these two numbers. Move out + and - 2 sd, and 78% of your shots will fall within these two numbers. Move out 3 sd, and 93% of your shot will fall within these two numbers. sd is a measure of variance and you want it as small as possible. Excel can compute standard deviation for you. Or you can google the term and get all sorts of math stuff. Enjoy.

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    20 to 30fps is good ammo in my book, but the game is to get them as close as possible.
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    Default dont give up matt !

    Pretty much the lower the standard deviation ,the more consistant your ammo ! Statistics is interesting when you see how it is applied in a manufacturing situation ! The real advantage I think is if you were making a large quantity of ammo and want to test it's consistancy , standard deviation lets you see from a relatively small random sample that your ammo is constistant and that the result is "normally distributed " ! If the distribution is normal it means that about 99.6% of the ammo you made will give the same results as your sample did ! It is kinda hard to explain and I havent used it in years !! Kevin
    Little bullets might expand ! But big bullets never shrink !
    When you swim with big fish try not to act like bait !

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    Default get a good statistics book

    Get a good statistics book for next winter ! Try Statistical methods for managers by , Hitoshi Kume ! By the way itsound like your ammo is pretty consistant !
    Little bullets might expand ! But big bullets never shrink !
    When you swim with big fish try not to act like bait !

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    Well my chrono calculates what my standard deviation is. So that is cool. No math on my part. I was just wondering what the number meant. Thanks for all the info on it!

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    SD is a number that tells you how likely the next round that you shoot is going to hit that exact average of the previous rds fired. A sd of 0 would mean the next rd will be the exact average. A sd of 6 or 7 means all of your rds are hitting pretty close to the average. As the sd goes up so does the chances that the next rd will hit average. Simply put, low sd means low velocity spreads and high sd means wide velocity spread. A sd number gives you a computed number that tells a rocket scientist how wide your velocity spread is. Unfortunately, it doesn't necessasarily relate to accuracy. But I'm not a rocket scientist, so this is my guess at sd.

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    Quote Originally Posted by shphtr View Post
    Intereting stuff, I'm going to use this five shot theory in my next test.

    I generally use four of each charge and go up in .3gr increments to establish a curve. I use a lot of bullets but I get good results and end up seeing which charge groups well along the way to max charge.

    Generally there are two charges side by side that group really good and I pick a load in between those two charges so I can rule out the +/- .1gr error in most scales and end up with consistent ammo.

    The article link above will save a lot a bullets and time while arriving at max charge in your particular rifle. You will still need to test for accuracy, generally I find that to appear at less than max charge.

    It's always nice to see an other approach and apply it. That's what makes this game so fun...

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    Just a little tidbit on Chronograph use. I got this information from Ken Oehler many years ago - if you're shooting subsonic (1,125 fps or less) you may want to use a blast shield/baffle placed half way between your muzzle and the 1st skyscreen. The reason is the shock wave can cause erratic velocity readings. This is kinda the reason why - When the bullet leaves the barrel say at 900fps it’s pushing a column of air in front of it that is going the speed of sound (1125fps) so that shock wave will at times trigger the start screen at 1125/1130fps with your bullet about a foot or so behind the shock wave. When the bullet crosses the stop screen the computer gets confused and you get to read its confusion thinking your chronograph just went titts-up. How often does this happen? Not often but when it does, it’ll probably do so for quite awhile due to “just right conditions” for the start screen to read that shock wave. I had it happen to me a couple times and that’s when I called Mr. Oehler and he advised me of this “phenomenon” and I’ve used a plywood baffle ever since on subsonic rounds and never have experienced irregular readings since (currently using a 4X2 plywood with a 1.5" slot cut down the middle starting from the top and going down 30"). Mr. Oehler advised there currently is no known method of manufacturing skyscreens that can remedy this problem.

  15. #15

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    Quote Originally Posted by JoeJ View Post
    Just a little tidbit on Chronograph use. I got this information from Ken Oehler many years ago - if you're shooting subsonic (1,125 fps or less) you may want to use a blast shield/baffle placed half way between your muzzle and the 1st skyscreen. The reason is the shock wave can cause erratic velocity readings. This is kinda the reason why - When the bullet leaves the barrel say at 900fps it’s pushing a column of air in front of it that is going the speed of sound (1125fps) so that shock wave will at times trigger the start screen at 1125/1130fps with your bullet about a foot or so behind the shock wave. When the bullet crosses the stop screen the computer gets confused and you get to read its confusion thinking your chronograph just went titts-up. How often does this happen? Not often but when it does, it’ll probably do so for quite awhile due to “just right conditions” for the start screen to read that shock wave. I had it happen to me a couple times and that’s when I called Mr. Oehler and he advised me of this “phenomenon” and I’ve used a plywood baffle ever since on subsonic rounds and never have experienced irregular readings since (currently using a 4X2 plywood with a 1.5" slot cut down the middle starting from the top and going down 30"). Mr. Oehler advised there currently is no known method of manufacturing skyscreens that can remedy this problem.

    This would only apply if you have the skyscreens close to the muzzle. If they are say 20 feet out I doubt very much the air has enough mass to reach the sky screens at anywhere near the energy level it had at the muzzle. In other words, it may be supersonic at the muzzle but being as light as air it will slow down rapidly and decelerate. The bullet will be well past what remains of it in a few feet.

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    Quote Originally Posted by Jack49 View Post
    This would only apply if you have the skyscreens close to the muzzle. If they are say 20 feet out I doubt very much the air has enough mass to reach the sky screens at anywhere near the energy level it had at the muzzle. In other words, it may be supersonic at the muzzle but being as light as air it will slow down rapidly and decelerate. The bullet will be well past what remains of it in a few feet.
    I apologize for not using the correct wording in my initial post and maybe misleading you with the term "blast shield" and pushing a column of air. I should have used the term sound wave shock wave, (don't know how else to describe it) which is different from the gas blast shock wave, which is what you're referring to. I believe the bullet travelling subsonic will pass through the gas blast shock wave at a distance 25 times its bore diameter. The sound wave shock wave will travel at the speed of sound and will always remain in front of the bullet as long as the bullet is subsonic. Technically I guess the bullet isn't pushing the sound wave, as the sound wave was created at the time of firing the rifle/revolver and is traveling independently for a lack of a better description. How far in front of the bullet the sound wave will be depends upon the bullet speed. This is the "phenomenon" that Mr. Oehler was advising me of, that I had never considered before that time. While talking with Mr. Oehler back then he also advised me that Lee Jurras, the man who started Super Vel, experienced occasional problems with his factory chronograph due to this sound wave shock wave and requested his assistance. The problem disappeared for good when they installed the baffle, which Jurras called a "muffler". I don't know how many millions of rounds Super Vel made but over the years the chronograph worked as advertised with the "muffler".

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    Quote Originally Posted by JoeJ View Post
    I apologize for not using the correct wording in my initial post and maybe misleading you with the term "blast shield" and pushing a column of air. I should have used the term sound wave shock wave, (don't know how else to describe it) which is different from the gas blast shock wave, which is what you're referring to. I believe the bullet travelling subsonic will pass through the gas blast shock wave at a distance 25 times its bore diameter. The sound wave shock wave will travel at the speed of sound and will always remain in front of the bullet as long as the bullet is subsonic. Technically I guess the bullet isn't pushing the sound wave, as the sound wave was created at the time of firing the rifle/revolver and is traveling independently for a lack of a better description. How far in front of the bullet the sound wave will be depends upon the bullet speed. This is the "phenomenon" that Mr. Oehler was advising me of, that I had never considered before that time. While talking with Mr. Oehler back then he also advised me that Lee Jurras, the man who started Super Vel, experienced occasional problems with his factory chronograph due to this sound wave shock wave and requested his assistance. The problem disappeared for good when they installed the baffle, which Jurras called a "muffler". I don't know how many millions of rounds Super Vel made but over the years the chronograph worked as advertised with the "muffler".
    I see what you're saying now. An interesting fluid dynamics problem. In essence there are two shock waves, the precursor that forms in front of the bullet and the one that forms from the expansion of the gasses at the muzzle in what is essentially a free jet type of expansion. Since the bullet is subsonic no sonic wave is formed from the movement of the bullet. The combination of all of this does result in a sonic wave as you describe. The magnitude of this wave is determined by multiple factors. The effect from a handgun firing subsonic is as you indicate is different from a sonic or supersonic event. However, the energy of these types of waves dissipate rapidly. I suspect part of the problem in testing at manufacturing facilities was the indoor nature of the facility which would lead to all sorts of interesting reflections of the waves likely making the problem worse. I speculate that the effect mentioned is likely not as pronounced outdoors. Thanks for the followup.

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