HS-6 and Primer Selection

I once worked up a load of HS-6 under the NOE 360-160-WFN boolit (3” barrel, .357 Mag). When I got to 10 grains, I switched from CCI 550 primers to Federal match grade magnum primers. The SD dropped to single digits. When I got to the max charge, average velocity was around 1,185 fps. Then, I put together 100 rounds with CCI 550’s and the same max charge - SD went to about 45, average speed was 1,225 fps, and one round went 1,300 fps. These were all metered out with a trickle charge. The primer cups show a range of pressure signs. I’m going to make a box of 50-100 rounds using both primers, to see if that different performance shows up again.
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Moral of the story: consider match grade primers when going to a max loading.