SD stands for standard deviation. 1 SD means that 68% of your shots fall within a certain spread. A load with a 2,700 FPS average and SD of 5 will have 68% of its shots from 2,695 to 2,705 FPS.
Primer flow happens for a lot of reasons. Nearly ever 700 I have seen has a little cratering. Shoot 6.5x47 or Lapua Creedmoor brass and tough primers are your friend, you will pierce primers at even moderate loads.
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