I live in "Aridzona" where temps reach close to 120 degrees in July and August.
I'm not sayin' it's safe to do so but I left my loaded Glock 21 in my trunk in my driveway all day long. It was 117 ambient temp that day and about 140 in the trunk. Nothing happened.
Wouldn't do that too often. and don't put the gun inside the main passenger area. Temps the local T.V. station did for the Milwaukee area got over 200 degrees F. inside the car. That's asking for trouble.
Things can become unstable in those kind of conditions.
You can always get a cooler with those frozen gel packs so you don't get any moisture in the gun if your that worried. But you will get condensation in there. As for rounds cooking off, Go too the mythbusters site on youtube. They cooked loose round off at the bomb range in a fire, it takes awhile for the primer to boil in fire to cook off. Basice rule of thumb is if you can stand the heat odds are your ammo can as well, but don't quote me on that.
I live in North Texas and have always had ammo in my truck. I'm not saying it is the suggested way of doing things, buy I always keep a couple hundred rounds and hearing protection in my vehicle. I am in sales, I travel, I make sure I have ammo for going to the range when out of town (it's a good way to keep myself entertained for a night on my trips). It gets pretty hot here and I have not had a single round go off.............yet.