Barrel heat and increased velocity across a string

so I was testing some loads today at the country club and brought the chrono. I usually leave it at home until I've found a good load and I'm simply using that to input into the ballistics calculator.

What I noticed today was on one of the loads the velocity started off at 2710 and climbed to 2754 by the 5th shot. This isn't the first time I've seen this. I'm assuming others have similar results? I tried waiting about 30 seconds between shots on my last group and had an extreme spread of 5. How long do you guys usually wait between shots when testing?
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