Preliminary Experimental results

Discussion in 'General Reloading' started by DaTeacha, Aug 19, 2010.

  1. DaTeacha

    DaTeacha Things are not what they seem. Forum Contributor

    Okay, I just got home and mowed some grass for the sheep and donkeys since the pasture is getting pretty thin. I only have a little time here now. More will follow after I get some chance to work with things this weekend.

    The experiment described in "Experiment coming up" is finished. The set with intentionally great distribution of case weights came in at 3.692 inches by initial measurement. I noticed a pretty wide scattering of the ejected cases compared to the others with this group also.

    The set with the random weights was pretty good by comparison, measuring a 2.387 with 3 obvious flyers ruining what would otherwise have been a nice cluster of holes in the paper.

    The final set, comprised of cases that I had grouped to weigh 93.2 grains +/-.1 was a little disappointing. It measures 2.256 at the widest and seems to be comprised of two subsets of what appears to be 9 rounds each. The other two are in there somewhere, but I don't spot them with a cursory examination.

    I will post pics of the groups this weekend. As a followup, I will resize/deprime all 60 cases and weigh each one carefully, reporting the results here. There may be more or less variation of the case weights in each group than I tried to have going in. I will not trim them or clean the primer pockets, just deprime/resize.

    At this point, there appears to be a slight advantage to weight grouping the cases, and there also appears to be a definitive advantage to eliminating the ones that are outside what I estimate to be 1.5 to 2 standard deviations from the mean weight of a group. I'll do the math later, too and see what it shows.

    For now, you guys can chew on these results as you see fit. Have fun!
     
  2. Browning

    Browning G&G Newbie

    Quick question, u mention 3 flyers in the secong grouping. are they incorporated in the measurement of the grouping or not. If not what was the measurement of the grouping without them?
     

  3. I've seen this same test done in just about every gun rag at one time or another. Their results mirror your own.
     
  4. samuel

    samuel G&G Newbie

    15,235
    36
    I have run tests with up to 120fps deviation and at 100yds shot relatively tight groups,(at least satisfactory for anything but competition) but when it went to 500yds and beyond,all hei* broke loose. ,,,sam.
     
  5. 338RUM

    338RUM G&G Evangelist

    I am glad you got results, I hope you can continue to refine your experiment to make it more accurate and inclusive, now take it one step further, weigh the bullets for consistency and retry the experiment to see if you get any better results!
     
  6. All the cases need to be the same length.
     
  7. DaTeacha

    DaTeacha Things are not what they seem. Forum Contributor

    First of all, mouser868, the cases were trimmed prior to chamfering and deburring and cleaning the primer pockets. For anyone else just coming into the discussion, please read "Experiment coming up" before you go further here so I don't need to keep explaining the steps I took. It just wastes bandwidth or something of the sort if I have to keep repeating things.

    I ran some numbers on my sample. There were 222 cases in the sample of PMC small headstamp cases used, not the 212 I reported earlier. There is a breakdown of how many were in each category elsewhere. The mean weight of the sample cases was 92.889 grains, which I rounded to 92.9 for calculation of the Standard Deviation. Presuming my 222 cases were a sample of PMC cases and not the entire population (there were a whole lot more than that made), I calculated the Standard Deviation of the sample and found it to be .45 grains. In a perfectly normal distribution, there would be about 68% of the population within one S.D. of the mean. In this sample, which I can only presume is representative of PMC small headstamp cases in general, fully 80% of the cases were within 1 SD of the mean. Further, all but 3 of them were within 2 SDs of the mean. Bottom line is that PMC cases are pretty well weight controlled from the factory. The number of exceptionally heavy or exceptionally light cases is pretty small, in this case 3 out of 222, or about 1.5%

    Pictures of the groups follow. The first one is the group shot using the cases with the widest range of weights. Measuring the farthest holes with an eyeballed center to center placement of my calipers gives a spread of 3.692 inches. As you can see by looking at the picture, there are 4 shots farther outside the group, circled in red. Throwing those 4 out leaves a cluster that is about 1.959 more or less.

    The second picture shows the results of using unsorted cases. Based on the statistical analysis above, of these 20 cases, it is likely that only one had a weight more than .9 grains higher or lower than 92.9 grains. In other words, it is likely that only one case varied more than about 1% from the weights of the others. This group measures 2.387 at the widest point. Throwing out the 3 holes on the top leaves a group with a vertical dispersion of 1.092 and a horizontal dispersion of 1.734. However, it is extremely unlikely that all three of the flyers were from extremely heavy or light cases.

    Finally, the third picture shows the results with all cases falling within +/- .1 grains of 93.2. It is worth noting that I had one failure to extract during the testing and that was with this group. I am personally intrigued by what appears to be a separation into two groups, an upper one and a lower one. Seeing no obvious flyers other than that apparent split, the group measures 2.256 inches across the widest spread. Measuring the lower half yields a vertical dispersion of 0.858 and a horizontal dispersion of 0.954. The upper set is 1.709 by 1.007.

    At this point, if I took the experiment no further, I would conclude that it is probably worth sorting cases, but only to weed out the ones that are more than 1% different in weight from the typical weight you find. In the case of PMC small headstamp .223 cases, that means more than about 1 grain away from 93 grains on your scale.

    I plan to deprime/resize the cases and weigh them more carefully to see if that yields any further insight. I'll share those results here. Based on what others have said, I will probably use a set of once fired cases - after I fire them - if I decide to do more experimental shooting. I'm going to start a thread to see what you guys think is most critical thing to do with cases in trying for that elusive one hole group.
     

    Attached Files:

    Last edited: Aug 20, 2010
  8. Ninja Piper

    Ninja Piper G&G Evangelist

    Teach, I found this program very helpful in analyzing group sizes. This may help you along with your experiment if you choose to refine it some more. I use this to keep track of the groupings with each different recipe I load. I take a picture of the target, input the shots, and save the file with the recipe. Just helps to have a visual depiction of how that load worked. I was wondering the same thing about if it was worth sorting cases or not. Thanks for posting your experiment!

    On Target Precision Calculator
     
  9. Browning

    Browning G&G Newbie

    god test Teach! Thanks for the hard work.
     
  10. DaTeacha

    DaTeacha Things are not what they seem. Forum Contributor

    Ninja, that looks like a very useful site. I've just bookmarked it and hope to work with it some tomorrow. Thanks for posting it.