Cooking Off

Regular readers of my unworthy scribblings already know that I have moved from snowbound Ohio ….

snow cthulhu 1

… to Sun-blasted West Texas.

desert between Odessa and Pecos

I got here in the dead of winter, so it was actually very comfortable for me at the time.  But the seasons are turning, summer is coming, and the temperatures are on the rise.  In a month or so, I expect that the midday heat will top 110 Fahrenheit (44 Celsius).

Thinking on the coming warmening, a memory floated up from the depths.  A reader once asked me if it would be alright if he stored his defensive handgun in his car during the hot times.  Would his ammo, perchance, cook off inside the baked interior?

I first determined that the inside of a car can’t heat up much more than about 50 degrees F (26 degrees C) hotter than the outside temp.  After that equilibrium is reached, and the car is radiating energy at the same rate it is absorbing heat from the Sun.

So how hot are we talkin’?  Worst possible scenario sees the inside of a dark car with large windows reaching 200 F (94 C).

bursting thermometer

Plenty toasty!  But is that enough to cause your ammo to cook off?

I reached out to all of the major American ammunition manufacturers, and received prompt and kindly replies from all except Winchester.  They were in agreement that modern ammunition in good condition only starts to cook off when the temperature reached 500 F (260 C).  As long as one loads up with recently purchased ammo, and cycles it out every year or so for fresh, then there is almost no chance that there would be  problem.

Stable and reliable as modern ammo might be in extreme conditions, the same cannot be said of most surplus ammunition.  Rounds manufactured prior to the 1960’s might well be primed with mercury fulminate, a chemical compound which is more volatile than the stuff which is used nowadays.  Not only are cartridges primed with mercury fulminate known as being corrosive, since the residue will react with steel to cause rust to form, but they also degrade fairly quickly with the passage of time.  Even if such ammunition was properly stored since it was first shipped from the factory it could still have become sensitive to high temperatures, and so should not be left inside a hot car on a blistering summer day.

burning car

So there you have it.  Buy new factory ammunition for your defensive handgun, and replace the carry ammo with brand new every year or so.  But you should be doing that anyway, shouldn’t you?

As for the ammunition engineers at Winchester, I am still waiting for your reply!

8 thoughts on “Cooking Off

  1. Trivia time.

    You are confusing primer compounds. Mercury fulminate is non-corrosive but degrades fairly rapidly over time. See your Wikipedia cite.

    Potassium chlorate is the “corrosive” culprit. After firing, the residue becomes potassium chloride which is almost the same as sodium chloride which is plain old table salt. Check out the ingredient label on a container of “salt substitute” next time you are in a grocery store.

    Potassium chloride is hydroscopic and after a few days will cause rust in a barrel. A cleaning patch wet with soapy water passed a few times through the barrel and then followed by a dry patch will remove the salt residue. Then clean as usual.

    I believe most manufactures quit using mercury in the very early 1900’s and started using the potassium chlorate formula. The U.S. military phased out the chlorate stuff in the early 1950’s and the rest of the world followed over the next 15 years or so.

  2. Whether or not you should leave your ammunition inside a car on hot day . . . you should NEVER leave your dog inside your car on a hot day.

  3. Unattended pistols stored in the trunk will heat up a little more slowly, and are more likely to be there when you return to the vehicle. Putting them inside a picnic cooler, or nested coolers, will likely keep them from getting too hot to handle. Same goes for photo gear. I have tried insulating the inside of the trunk lid, with mixed results.

    • Thank you for sharing!

      I was wondering if you can expand a bit on your efforts to insulate your car trunk. Sounds really interesting!

  4. I tried cutting foam building insulation to fit the recesses in the structure, but getting them to stick was difficult. I think that was partly because of the paint. Maybe spray on foam would work, with adequate masking of the rest of the car.

    • Spray foam will stick for sure. Make sure you never plan to sell the car because you will never get it off. I can’t say what Texas type heat will do to it though. I would suggest trying it in the 9th circle of hell first as an easy test run.

      I’ve never had much luck trying to get regular foams to stick because their surfaces seem to degrade and stick to the adhesive while the rest of the foam pulls away.

      Spray foam on the other hand is its own adhesive. I had to burn a set of clothes I was wearing when I sprayed that stuff because nothing, but nothing would remove it.

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