I have observed a phenomena haven't see mentioned here or elsewhere (probably my falut in searching) and it is the following:
with a frequency of one over 80-100 rounds, one of them results in a shot with a significant lower muzzle velocity ( around 2000 fps ), the primer seems 'inflated' towards the outside of the case head and the brass case neck is significantly dirtier. This results also that following shot is also always a little slower ( at least one of the slowest of the session, 10-15 fps below session average) then the second after is back to 'normal'
Recipe: Alpha OCD, CCi450, N150, B105, 2815 AVG, SD 5 or less over 30 shots, ES 25 (exluding the 'flyer' from stats). Primer seated 2-3k below flush, little crush in the primer pocket.
My best bet would be either a random problem with primers, or powder polluted by something ( cob grains left in the brass? ), but I woudl hear from the dasher gurus, even if I don't think is caliber specific. Thank you.