[Unclenick]

Two possibilities. One is that you were too close to the chronograph. I always use 15 feet now. Too many folks trying to make 6 feet work get false highs or lows from muzzle blast.

Second possibility is that with the case more full, the primer developed enough pressure to unseat the bullet before the powder burn caught up, so it was like shooting in a bigger case and got lower peak pressure. Usually the velocities are inconsistent when that's happening.