Introduction
The seating depth of a primer into a pocket is a hotly debated topic. There are various methods but no clear data to aid in the shooter's decision regarding what to do with primer seating. The purpose of this testing is to provide data on the topic to help shooters decide how to prime their cases.
Method
The test involved shooting 5-shot groups at 100 yards with varying primer seating depths: flush with the face of the case head, .003”, .006”, .009, and .013”. The primer pockets were examined with calipers and the pocket depth of the 6PPC Norma brass used was .1220” among all pieces of brass and the primer utilized was a CCI BR-4 which has a height of .1135”. This means that the primer would seat to the top of the primer pocket if it were seated to .0085” (the equation is primer pocket depth .1220” minus primer height .1135” equals seating area .0085”). Therefore the primers in this test seated to .009” are the closest representation to SAAMI specifications and the method whereby you seat to the top of the primer pocket. The primers seated to .013” represent primers that are significantly “crushed.” When seating to .013”, it was obvious that the seating pressure was significantly higher than any other seating depth. I do not recommend crushing your primers given that a primer may ignite while crushing. I wore safety glasses during the primer seating and recommend same. It is worth noting that different primers have different heights, for example, CCI 450 is .1220”. The groups were shot in differing order each time, i.e., flush was shot first on the first occasion, .003” was shot first on the next occasion, etc. This pattern of changing the order rules out “order effects”.
Results
An analysis of variance (ANOVA) statistical test was applied to examine the differences between the groups of varying primer seating depths. For the group size data, the ANOVA revealed that there was a statistically significant difference (p=.015) in the various primer seating depth groups: Groups Count Average Flush 5 0.4282 .003" 5 0.2682 .006" 5 0.2882 .009" 5 0.1992 .013" 5 0.4321 The primer seating depth of .009” had, on average, the smallest groups and aggregated at .1992”. .009" was statistically significantly lower than flush and crush. ANOVA was conducted on the velocity and standard deviation data and revealed no statistically significant differences in the primer seating depth groups. Although not statistically significant, it is notable that the .009” seating depth had the lowest average standard deviation. A review of the percentages of the shapes of the groups revealed that that groups shot with the .009” primer seating depth showed the greatest consistency with 80% of the groups showing a circular shape. The primers seated to flush showed the greatest variability in group shape. The crushed primers showed the most fliers.
Conclusion
According to this test, the method for seating your primer should follow SAAMI specifications and seat the primer to the top of the primer pocket. Seating flush with the face of the case head or seating that significantly crushes the primer may lead to very poor outcomes. In situations where primer pockets and/or primers vary considerably from SAAMI specifications, it is recommended that you measure your primer pocket depth, primer height, and set your primer tool to seat the primer right at the top of the primer pocket.
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5 окт 2024
