In general, it's not a problem.

I rotate carry ammunition once a year. And it is exposed to some great heat in the vehicle where it's left all day while I'm at work. Never a problem with any caliber, gauge or brand. I'd say 30 years in Arizona has offered me some experience. YMMV.

Keep your ammunition in reasonable temperatures away from humidity. It should be fine for a lifetime or two. Or three.

I've shot a lot of ammunition from the 40's and 50's. It all has been 100%. I found a few boxes of 12 gauge bird shot on a rafter in an outdoor laundry room in a house I leased and the box tops were cut off. A little research indicated they had been there for some 25 years or more based on when those shells were last manufactured. I wiped the dust off the shells with a damp rag and bagged a few dozen dove and quail with them.

The only ammunition I have found suspect is .22 LR. Had some bad luck with that. Several brands put on a closet shelf, 6 months later was getting 10-15% failures to fire almost across the board. Weird. I don't know if that's a thing - but I don't own any .22 platforms today because of that experience. The few I've had since, I cycled all the ammo out every 6 months.

A lot of people are getting away with it, but bear in mind that accelerated aging tests are run at temperatures not much higher than a closed car in the summer. I have read of testing at 149 and 165F.

I remember my father in law had some old ammo and it would cause issues, but like I said it was very old. The coolest thing about it was you could actually see the bullet come out of the barrel and it would only go about 20-30 feet and hit the ground.

quote:

Originally posted by patw:
I remember my father in law had some old ammo and it would cause issues, but like I said it was very old. The coolest thing about it was you could actually see the bullet come out of the barrel and it would only go about 20-30 feet and hit the ground.

Yeah, I'd say that qualifies as "issues."

Depends on the use.

DoD has started to add the requirement for "temperature insensitivity" or "thermal stability" to it's ammo requirements/testing for small arms. Most of the info is on DTIC but isn't necessarily releasable. My understanding is that this concern started when guy noticed issues with the sniper rifles in Iraq. However, I've also seen thermal stability test data on a pistol round. It appears that exteme temperature variation (160 degrees) had an impact on velocity, but not chamber pressure (very strange) but the way the data was presented was less than optimal. The round passed testing so the variation was not enough to cause concern.

If you look through the civilian long range precision shooting community you can find discussions and data on temperature and ammo and Temp it's relation on bullet flight.

Back towards your question, I know a couple LE agencies that keep ammo in their vehicles year round with out reported issue and be kept our ammo in the Trucks in Iraq without issue.

What you should do is take some ammo from the same batch and lot and test them for velocity and accuracy, one group stored at constant temperature, and the other after a period of time in the conditions you describe.