This happens quite a bit during my load tests.
I'll have a variety of different loads to chronograph and the first shot in a string will be faster than the rest. In addition to a speedy first shot, consecutive shots get slower. For instance, the velocities of one load tested today went 1412, 1345, 1343, 1350, 1340, 1343, 1329, 1331, 1315, 1310.
Doesn't happen every time but often enough that it's noticable and makes me wonder if it's my chronograph or there's something else going on?
Anybody else notice the same phenomenon?