I knew it was going to get pretty hot when I moved from Ohio to Texas. It isn’t unusual for the temperature to climb past 100 degrees Fahrenheit (37 Celsius).
One of the questions my students would ask is if they could store their defensive handguns inside a car during the summer months. Would it get hot enough for the ammo to cook off?
Some years ago, I reached out to all of the American ammunition manufacturers that I could think of. All of them had the same response. As long as the ammunition is brand new, factory fresh product that has been stored in a cool and dry place, then the primers should not cook off unless temperatures exceed 500 degrees Fahrenheit (260 Celsius).
That is all well and good, but how hot does the inside of your car get when parked in the sun with all the windows rolled up?
This study by the Department of Geosciences at San Francisco State University has some interesting data. It would appear that the interior temperature of a car can rise above the outside temperature by as much as 50 F (27 C). I’ve also seen studies where the interior temperature could rise as much as 60 F (33 C). Any way you look at, the interior of your car will not come anywhere near the temperatures needed to cook off new, properly stored ammunition.
The one thing that has to be stressed is that we are talking about brand new, factory fresh ammunition. High temperatures will cause the components of ammunition, the propellant and primers, to degrade. Give it enough years and the ammunition inside your defensive handgun might well become volatile.
Note that I said “Give it enough years...”
Your defensive arm is subject to a great deal of harsh environments, starting with increased humidity just by being carried close to your skin. That is why I strongly suggested to each of my students that they change the ammunition in their gun for new every year. Take the old stuff to the gun range, shoot it off for practice, and buy new boxes for carry. I do this on my birthday, sort of a gift to myself.
Also note that one should not be using old surplus ammunition for defensive purposes. Reserve that stuff for training, and rely on newer for defense. If you do that, you should never have to worry about ammo cooking off in your car.