First shots can go either way. Typically the first shot from a clean barrel is lower than average.
SAAMI procedures require two or three "warmers" immediately prior to their 10-shot tests.
Watch out for powder position during firing. The "SAAMI Twist" with careful handling to assure that powder ends up at rear is essential. I've seen extreme changes with powder position, but haven't seen as many changes with first shot.
KenO
