Causes in order of probability:
1. Shooter's desire to head for air conditioned car.
2. Change in ballistics due to powder getting warmer.
3. Expansion of barrel.
More seriously, the ambient temperature will have a noticeble effect on the ballistics of the particular load. I would expect this effect to as large or larger than that due to the barrel heating up. The Brits used to put up two loads for a rifle, one for decent temperatures and one for the hot tropical climate.
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