Most ammunition is manufactured to perform safely in temperatures up to 85 degrees. Once the temperature starts rising there is a very good possibility that the pressures rise also. These pressures can rise high enough that damage can happen to your firearm and/or yourself.
There are two major causes of cartridge temperature increases: ambient temperatures and chamber temperatures. Ambient temperature can change with air temperature and exposure to sunlight. Chamber temperature can change with air temperature and firing ammunition.
Ambient air temperature can be affected by the air around the cartridge, the heater in your vehicle, your body heat, sunlight on the cartridge, etc. For instance, if you lay your ammunition on your vehicle dash with your defroster running and the sun shining through the windshield. This could heat your ammunition to a high enough temperature to cause damages.
Another, often overlooked situation, is where several rounds of ammunition have been fired in your firearm. Then you chamber a round and don’t fire that round immediately. The heat from the hot chamber will heat up the loaded ammunition to a dangerous level.
