Since its so hot in Iraq, do bullets explode?

Not at all. Often times it is hotter inside the vehicles (if they do not have AC) and they don't ignite there.

When a round explodes due to heat it is called a "cook-off" and can happen, but it doesn't happen from just the temperatures from the sun. Although cook-offs are rare, they are more likely to happen in automatic machine guns that have a closed bolt system, due to that, the majority of machine guns in the world have an open bold system, but they can still have cook-offs. I have heard of cook-offs happening in the M60 (yes, that is an old gun) and M240.

The short answer is no. The heat generated by the sun will not heat up the barrel or chamber enough to cause an ignition.

Only after high volumes of fire over a prolonged period of time will the weapon become hot enough to ignite the primer. The sun can effect this process by causing the ambient heat to rise, thus slowing the rate at which the weapon will cool; leading to a faster "cook off" of the round in the chamber.