One common reason for measuring/comparing primer brisance is to determine whether primer
weight can actually be used as a valid surrogate for primer brisance. The primary benefit or reason for doing this is for the purpose of sorting primers. It is very easy for anyone with a good analytical balance to weigh primers accurately, whereas it is not so easy to directly measure brisance. For such endeavors, the assumption has to be made that differences in primer weight are due to a difference in the amount of priming compound (i.e. that cup and anvil weights are very uniform -
#1). It is conceptually not much different than the idea of sorting cases by weight as a surrogate for volume. An additional assumption must also be made; any variance observed in the brisance test output must necessarily translate to a proportional change in velocity (
#2). If either of those assumptions is invalid, then it will be difficult, if not impossible, to effectively correlate primer weight with brisance.
I really like John's rather clever and out-of-the-box-thinking approach to adapt the ShotMarker for this purpose. This is the kind of thinking that can often lead to new and worthwhile discoveries. From his results, it doesn't appear as though there was any significant difference detected between the lightest, heaviest, or random distribution of primer weights using this approach.
Frankly, I am not too surprised given the marginal difference in the two sets of primer weights (0.12 gr total range, or ~7.8 mg). The weight difference between the High/Low primers in John's test translates to only about 2.2% of the
total primer weight, which is a very small weight difference in light of assumption #1 (above). The weights of primer anvils and cups would have to be extremely uniform in order not to invalidate the assumption that any difference in primer weight is solely due to priming compound. In the past, I have weighed a few boxes of Fed205 primers and I came to a similar conclusion. In my hands, the total High/Low weight range was 5.9 mg (~.09 gr), which was about 2.5% of the total primer weight. In other words, detecting a difference in primer brisance that is proportional to a difference in primer weight, which is of itself only a very small fraction of the total primer weight, is asking a lot.
In any event, I give John full marks for coming up with a novel approach to measure primer brisance using the ShotMarker. However, you might want to be careful tossing these results out in public forums like this, John. I haven't yet purchased a ShotMarker and if Adam hears about your test, he is liable to start advertising and selling them as both an E-target AND Primer Testing Apparatus with a commensurate increase in MSRP, which would be problematic for me.
The other item of interest that this report has brought back to my attention is with regard to assumption
#2 (above). Specifically, no matter what method is used to quantify or compare primer
output (i.e. brisance), we must assume that any change in primer output is accompanied by a proportional change in velocity. If not, there is no reason to even conduct such a test. Because of my background as a scientist, I am usually a strong advocate for isolating a single independent experimental variable as cleanly as possible, so as to minimize the influence of other variables on the interpretation of the results.
However, this is one case where I am not sure that is the best approach.
The sole reason for measuring primer output is so that we can either support or disprove the notion that primer output is proportional to primer weight. The reason we wish to do this is so that we can then use primer weight as a simple and efficient method to sort primers and improve precision on the target. It's a very good reason for wanting to correlate the two factors, IMO. Arguments in favor of isolating the primer output variable as cleanly as possible are that other variables including charge weight, case volume, and bullet weight become important and may negatively influence interpretation of the results if velocity is used as a "downstream" readout for primer output. I view the situation in a slightly different way. Certainly, variance in charge weight, case volume, and bullet weight can affect velocity, making it more difficult to specifically isolate the effect of primer brisance on velocity. However, my thinking is that we can
never completely remove these additional variables from the equation, at least, not if we want to actually put bullets into a target downrange and measure group size.
That being the case, I find it difficult to envision a better readout than measuring the velocity of the exact bullets you intend to reload. Yes, we certainly need to consider charge weight, case volume, and possibly bullet weight as additional variables in the equation when attempting to test primer brisance, but we generally try to keep any variance for those factors as tight as we possibly can, and they can never be removed from the actual shooting equation, anyhow. My argument for using velocity as a readout for primer brisance is that if we hold the tolerances of case volume, charge weight, and bullet weight just as tight during load development as we would when loading rounds for a match, that is the best we can ever do. The effect of those other variables won't be zero, but it should be as consistent from round to round as we can ever realistically make it. In other words, it's all about limiting sources of error. Under those conditions, we either will, or will not, be able to detect a difference in average velocity as primer weight changes. If we cannot detect a difference in either average velocity or the associated ES/SD value as a direct result of a difference in primer weight, then primer weight variance is probably not the limiting source of error and it becomes almost impossible to make any claims regarding any perceived beneficial effect on the target when using primers sorted by weight. For that to happen would require the existence of some velocity-independent mechanism by which primer brisance could affect precision, which seems pretty much like magic to me. Some modern chronographs are certainly more than capable of measuring velocity accurately down to a few fps, which ought to be sufficient to state whether primer weight actually did, or did not, have a measurable effect on average velocity or ES/SD. Obviously, the easiest way to approach such an experiment initially is to use primers from the Highest/Lowest weight extremes within a given Lot, as John did in his report here. I have been intending to do this test myself for some time, which is why I weight-sorted primers in the first place. However, sometimes life has a habit of getting in the way of a good shooting experiment and I simply haven't gotten around to doing the test as yet. When I do, I will certainly report the results here.
In any event, I appreciate John sharing his test results with the rest of us. Very interesting report!
