even before i read the oven article, the heat here had me thinking. is it possible for the heat that builds up in a car on a hot day to cook off a round in a loaded gun?

In a very old reprint of a 1950's NRA article, designed to help firefighters deal with ammunition fires, the NRA determined that "cook-off" temperatures were in the 400 degree range (if I remember correctly). Since a car interior can quickly get up to 130-140 in the summer, but probably won't go much higher, I doubt it is a concern. However, those temperatures are probably not good for long-term ammo storage and may decrease reliability after awhile.