first round slower then rest

[love2shoot]

Noticing a pattern with the heavier projectiles in my 308 tokerav project. I have been running strings of five through the chrono for each load to check velocities before going on to other testing with the loads. seems the first round on each string is 50 to 120 fps slower then the rest of the string. had one today, first round was about 70fps slower then the rest. the other 4 rounds had a spread of 6fps (yes a SD of 3). trying to figure out why this is happening.