I get this asked a lot, especially in states where you are allowed to conceal and carry and you store a pistol in your car. This can be a problem, in the summer you car gets hot inside very quickly. While it will never reach over 400 degrees, the temperature required for ammunition to explode, it can still cause other problems.
With ammunition itself, your main concern is humidity. There is a chance with factory ammunition that moisture can get inside the round and cause it to not function. You will definitely have to worry about this if you reload your own ammo as you are less likely to be able to seal the round like a factory can. With firearms, you have to worry about the humidity issue. Moisture can get inside the gun and cause different parts to rust. This can cause the gun to not fire or malfunction in general. You’re other concern, with storing a firearm in any in a vehicle, is security. It is very hard to secure a firearm in vehicle properly and with vehicle break and enters, you should be concerned about long term storage of your firearm.
Ben