It's an interesting experiment to add temperature as a variable to ammo velocity, and the results aren't what you might expect.
The guys from the Wound Channel performed an interesting experiment with ammo temperature as it relates to velocity.
To test the "hot" versus "cold" velocity of ammunition, five bullets were left in the 90-degree heat for over an hour while another five were left in an air-conditioned car for the same amount of time.
See which ammo flies faster.
While a few commenters mentioned that a control of firing ammo that had been in room temperature would have added even more to the ammo test, it's certainly interesting to see that the hotter ammo had the greatest average velocity.
Moral of the story: Don't be afraid to let those bullets bake in the sun!
Within reason, that is.