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.

[rumlover]

I'm not sure why that happens on a cold bore, but I've found it to be consistent across the sub loads I've tried in 308 and 300W. I started swabbing the barrel with a very light coat of CLP and the problem went away (or at least the difference was within the standard deviation of that load). Also, if you let the barrel cool for 10 min or more between shots, you would see less deviations.

My guess is that a cold bore diameter is more/less than a warm bore diameter, but that may be too simple of logic (warm steel expands aka the lands expand into the bore rasing the pressure).