INSULATING YOUR AMMO
Having spent a considerable amount of my life in southwestern New Mexico, where the summer heat is, well hotter than heck. I've stumbled upon a few ideas that should help you deal with the effects of heat on stored ammunition.
First and foremost, never ever leave your ammunition exposed to elevated temperatures for any length of time and do not leave ammunition out in the burning sun.
Basically, cold seems to have no effect whatsoever on either the powder or primers, while heat causes a breakdown of the chemical components that make up both the powder and primer. So if you are not able to store your ammunition in the house, then a good rule of thumb is this: if the heat or humidity (outside or inside) is causing problems for you (physically), then it is also going to cause some problems with your stored ammunition.
What this means to you is that the trunk of your car, your garage or storage shed with non-insulated walls (includes the roof) are not the best places to store your ammunition. Unless you take some basic steps to help prevent your ammunition from overheating, you're likely to experience, at the very least, misfires when you need your weapon fully operational.
This is especially important during the summer months when the sun is at its peak and can raise the temperature inside to the point where chemical breakdown occurs. What point would that be? Well, I honestly do not know; however I have heard stories/rumors around the campfire of individuals finding loose rounds in their truck (or car trunk) and what they experienced were either a lockup or a misfire.
By lockup, I mean that the weapon fired, the bullet left the chamber and accomplished its task, but the action locked up and would not function for a follow-up shot, neither of which is a good thing. Now what can be done to correct this? Well, the best option is to store your ammunition in a well-insulated area; in other words in your house. If you are unable to do this for whatever reason and have to store your ammunition in another area, try using either a non-functioning refrigerator (they are extremely well insulated and have adequate storage space) or a large insulated cooler (not the Styrofoam ones). The insulation in either of these items can effectively insulate your ammunition from the effects of excess heat.