Witch Doctor: Effects of Primer Seating Depth and Anvil Compression on Precision: UPDATE!

Introduction
In the last primer depth test, we found that primer seating has a major impact on outcome (e.g., grouping). It was clear that a seating depth that placed the primer into the primer pocket at .009” (SAAMI guidelines recommend .008”) was ideal given that the aggregate of the five 5-shot groups was .1992” relative to flush .4282” and .013” .4321. When you examine the anatomy of a primer relative to a primer pocket, you can see that the anvil protrudes beyond the primer cup and when seated at .009” for the particular primers used in the previous test (BR-4), the anvil is compressed .009” but the primer cup itself has .004” clearance from the top of the primer pocket. When the primer was seated at .013”, it was flush with the top of the primer pocket which would have caused maximum compression on the anvil which degraded precision significantly. While I suspect that the anvil compression is the major factor in precision, this follow up explores whether the primer cup itself and its location in the pocket is a factor to be considered. For the purposes of this test, I utilized a CCI 450 primer which has a height of .113” so when seated at .009”, the anvil is fully compressed. If the compression of the anvil is the sole factor in precision, then the CCI 450 seated at .009” should exhibit poor performance similarly to the BR-4 when the BR-4 was seated to .013”. However, if there is an interactive impact of the primer cup height itself and the greater primer cup height somehow compensates for the fully compressed anvil, then the fully compressed anvil in the 450s should perform similarly to the BR-4s which have a remaining .004 clearance to the primer pocket bottom. I realize that these are different primers so there is more than one variable varying at the same time but we will test to see how much variation we could expect from 450s and BR-4s in order to understand how much of an effect that variation would have on the outcome of the main test.
Method and Results
Test 1:
An initial test was performed to assess whether 450 and BR-4 primers would show substantial differences in performance. I loaded 15 rounds with the 450 and seated them flush to the case head to represent no anvil compression. I used the exact same powder, load, bullet, brass, and loading practices as the first test with the BR-4s. I then compared the averages of the three 5-shot groups with the averages of the 5-shot groups from my first test. I realize that this is not a direct comparison because different days can have different atmospheric conditions but I wanted to have at least a ballpark estimate of just how different these primer would perform so that I can estimate how much of a confound this would be with the next larger test. The data showed similarities:
Primer Velocity SD Group
450 3338.3 13.3 .319
BR-4 3352.5 16.4 .387
The point of impacts and group shapes were very similar. Velocity averages were within 15 feet per second, both showed double digit standard deviations, groups were in “the threes,” point of impact was near identical, and group shapes were more similar than dissimilar. Collectively, when factoring in all of the data, the 450s and BR-4s are behaving similarly and I would not expect to see any major variations in their behavior if all else was the same.
Test 2:
50 rounds were loaded with 29.4 of N133 (note that a new lot was utilized for this test relative to the last test because I ran out of the powder from the last test; however, the year is the same but it is a different lot), Paul Parosky’s Patriot bullet seated .013” from the lands, and Norma 6PPC brass annealed and internal necks moly coated. The only difference is that 25 rounds had the Br-4s seated .009 into the primer pocket and 25 rounds had the 450s seated .009.
The same BAT Neuvo rifle was utilized for testing. All rounds were shot on the same day so that atmospheric conditions were relatively constant.
The data showed that the 450 and BR-4s performed the same:
Primer Average Velocity Average SD Average Group
450 3326.2 9.06 .2723
BR-4 3327.2 11.72 .2901
There were no statistically significant differences between 450s and BR-4s on any of the variables.
Conclusions
Building on the previous findings that showed that the anvil compression of .009” was ideal relative to other levels of compression with .003" variance in each tested condition, this test showed that primer cup size and the location of the primer cup in relation to the top of the primer pocket did not have an impact on precision. In conclusion, the anvil compression is the key variable and it is recommended that you compress the anvil at SAMMI specification (.008”) or .009”. Measure your primer pocket depth, primer height including anvil, and primer cup height to ensure that you are seating at the ideal depth and anvil compression.