ES and SD

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Vaughan
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Re: ES and SD

Post by Vaughan »

stokesrj wrote:
rick137 wrote:I certainly did miss the point but not your reply since you posted it twice.
Hmm, wonder how that happened :)
I think it's called "emphasis".... :lol: :lol:
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rick137
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Re: ES and SD

Post by rick137 »

Upon reflection, I realized that my error was not realizing the independent variable, i.e. the mass of the powder, was not constant. Therefore, the ES and SD of the dependent variables, the horizontal and vertical offset of the center of the bullet hole from the "shifted" aimpoint" do not have their usual interpretation. My mistake was in thinking the ES and SD referred to geometry of the bullet holes and not the muzzle velocity. Live and learn. One would expect an extreme spread in muzzle velocities with the smallest for the least mass of powder and the largest for the greatest. An SD can be calculated for any numerical data set but would the handloaders please explain its significance to me in this situation.

An alternative presentation of the data would be a pair of plots. The first would be muzzle velocity versus mass of powder. I presume the ideal would be a set of points through which a smooth curve could be drawn. Smooth in the mathematical sense of all points lying on the curve so that curve itself has no discontinuities in its derivatives. The curve could be linear or non-linear but being smooth would indicate the powder was homogeneous in its physical and chemical properties. The second plot would contain two curves as a function of powder mass, one for the horizontal offset of the bullet hole from the aimpoint and one for the vertical offset of the bullet hole from said aim point. I presume the objective is locating the inflection point in the curves for that represents the mass with the minimal, if not zero, magnitudes of the horizontal and vertical flexural modes of barrel vibration when the bullet exits the barrel.

Do the horizontal and vertical flexural modes have a minimum at the same mass of powder? An interesting question.
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Corjack
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Re: ES and SD

Post by Corjack »

ES can not always be just powder. I installed a new barrel, and set the headspace at about .004. Then the bolt handl broke off, I checked a spare bolt, but it had a headspace of .010. I went ahead and shot a match, and did ok, but when I then checked the ES, it was up in then 20s. I rehadspaced barrel, to give me about .004-.005 headspace with the exact same lot of ammo, and the ES went down to single digits like it was before.
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rick137
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Re: ES and SD

Post by rick137 »

Corjack:

Right on. A very large number of variables. By the way, in your post that started the discussion the SD in the link was to a situation where all of the significant independent variables, such as the various components of ammunition, were nominally constant. But nominally constant is not absolutely constant due to such as the manufacturing tolerances in the properties of the powder and the mass of powder loaded plus the finite accuracy of any measurement.

In the situation stokesrj described, I presume he was doing a load test and thus one of the significant variables affecting both muzzle velocity and accuracy was not constant by design. With respect to interior ballistics muzzle velocity is a dependent variable dependent on the entire internal ballistics and thus affected by many factors besides the mass of powder, one of which you described. However, with respect to external ballistics muzzle velocity can be a significant independent variable.

Until enlightened by the handloading gurus my opinion is the the SD of the muzzle velocities in a load development scenario is statistical nonsense, devoid of meaning. Unless the SD is with reference to the powder mass that gives optimal accuracy.
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stokesrj
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Re: ES and SD

Post by stokesrj »

In the scenario I described the meaning of both ES and SD are nonsense since by design, a ladder test, each of the 8 shots varied in powder charge by .3 grains. The reason it was posted was to show the complete lack of correlation between muzzle velocity ES and SD and the resulting accuracy or as you would call it precision. I find it absolutely amazing that a sub MOA group at 100 yards can be achieved with and ES of 168 fps and SD or 61.4 fps which are huge numbers, far more spread than you will ever see in any half way decent ammunition.
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stokesrj
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Re: ES and SD

Post by stokesrj »

I'm even further amazed with a five shot group I fired yesterday using a single charge from this ladder test at 400 yards. The average muzzle velocity was 2598, ES 18, and SD 8.2. Tell me the statistical probability of four of the five shots going through the same hole at 400 yards. Simply amazing and surely an unrepeatable phenomenon.
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Re: ES and SD

Post by rick137 »

Trying to ascribe meaning to nonsense does sharpen one's understanding and that is worth the opprobrium of the masses.
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Re: ES and SD

Post by icebug »

You'll have to repeat that Bob.
Next time you put all 5 shots through the same hole :mrgreen:
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