Its commonly understood that ammunition that is hot will propel a bullet to a higher velocity than those at colder temperature. In fact, the 1:7 twist rate for the M16 was chosen to be able to stabilize the tracer round at sub-arctic temperatures.
The Wound Channel did a short experiment using ammunition left in the hot sun versus those kept in an air conditioning vent. The difference of only 100 degrees is nearly 100 FPS, which with a 5.56 can be a major difference in lethality, especially at longer ranges. Its good news for our boys in the Middle-East who are plagued by high temps and something for them to look out for when up in the Afghan mountains.