Ammo sitting in hot vehicles

[TexWeatherby]

I had been wondering about this, since I have been leaving a pistol in a hot truck at work, all day, every day, all summer long with 100 degree days (probably 120F+ inside). Finally, last week I asked the company owner (who is a friend of mine) how he feels about concealed carry inside the building (company manual written by the lawyers says no weapons). It was basically, "just don't let anybody know, or everybody out in the shop will be packing." So, that conversation solved that problem, but I still wanted to know the real implications from heat on ammo. What I found is that heat is not that harmful (but higher pressures if you fire the heated ammo while it's still hot); the real enemy is humidity. Here's an article about it.
https://www.usacarry.com/safe-store-guns-ammo-hot-car/

[Grouchy]

Good info!  I swear it gets 400 degrees here sometimes. 

[danno50]

It's probably safe to leave your CC weapon in the car or truck in an employee parking lot all day, in the heat, but I, like the author, don't trust public parking for long periods day or night. Sometimes these lots offer a sense of protection with a security guard patrolling a large area in a small electric vehicle  . But auto break ins can happen anywhere, day or night even if you're parked in an area where you have a lot of walking traffic. I worry less about the heat than the brazen meth heads or sober pro thieves looking for opportunities.   

[kmoore]

It makes a good question. But, I have as many co workers have left ammo in car trunks, plastic boxes, factory cardboard, metal, in magazines, in vehicles such as cars, SUVs pickups and boats. I bet many other cops and military have also.
And when taken to the range or else where they fire.
As for weapons: The departments 12 ga and .308's sat in the gun locks inside the vehicle or in trunks. I had both, all of them were fitted with limb saver recoil pads. Those pads melted from the heat, after a few years they got replaced. Ammo was shot up normally on a yearly qualification.       
Another note not asked: Because motor officers and like me boat operators may have the service guns, ammo and spare ammo exposed to rain all day. How does water affect ammo?
I did a test with 38 spl ammo both factory and handloads. I keep 24 rounds under water for 5 minutes. I wiped them off and shot everyone without any problem.
My grandfather worked on a dredge in the Columbia River many years ago. According to my father a sealed metal box of M1 3006 ammo once was brought up. It was marked 1940 something AP ammo. I used the key to open the box, just like a spam can in 2015. The ammo looked ok, I fired a bunch clips of it in my M1. All fired as should. (AP stands for armor piercing). I gave the opened box and 2 bandoliers to a friend that owned a gun shop to display.             

[truck driver + 2]

One reason not to load ammo to max velocity. Since I do a lot of precision handgun target shooting I can tell you temperature will have an effect on velocity and pressure, .22 rimfire especially. When cold it won't reliably cycle a semiautomatic target handgun, there are powders we use in centerfire ammo that is very temp sensitive and will have the same reaction as .22 rimfire.
Ammo loaded at sea level and used in higher elevations will also react differently. I've gone to matches with .45acp ammo that worked great when tested at my home range only to have cycling problems when shot at a match. This past year I had to change the recoil spring in my .45 on the line during my relay at Camp Perry during the slow fire portion of the match. My ammo wouldn't reliably cycle my gun with a 12lb recoil spring. I try not to leave ammo in the cold or the heat if possible. One reason to use factory ammo for a concealed carry handgun.

[TexWeatherby]

+1 on that. For daily carried defense ammo, exposed to heat and humidity (including close proximity to a possibly sweaty body), I think it's a good idea to replace it with fresh ammo every year or so and just shoot the ammo that's been kicking around in all that.

[Big Muddy rancher]

Don't used WD-40 on ammo.

[Grouchy]

I just changed my ammo after mowing grass in 105 heat index. 

[zoniezonie]

Depends on where you live , we don't have the humidity here in Arizona but we sure have the heat. won't comment on humidity but the heat can and will definitely affect ammo, seen ammo breathe and smell while stored in old military ammo cans left in metal bldg's in the sun.  Generally most people here keep ammo in the house or garage or insulated cool place.  Wouldn't be a bad idea to have an under or above ground ammo bunker covered with dirt  and shade trees for insulation to keep it cool. At least here I can't leave ammo out sitting on the tailgate or any other place open to the summer sun ammo just gets too hot even to touch sometimes   I have to keep it in the shade as much as possible.  I do like to test loads when at or near max charge weights  when it's pretty warm out.  I've stuck enough bolt's thru the years to know heat and powder don't mix real well.  As temperature insensitivity as some of these newer powders are suppose to be imo there is a limit on just how hot you can get this stuff and be practical.  I keep my ammo covered out of the sun as much as possible.  I've melted plastic camera in the back seat, bic lighter explode on the dash , melted all of one of our boats plastic  electric panel switches  get soft and deform luckily our other boat has old school metal switches just can't touch them in the summer sun,  so we carry beach towels to lay on the vehicle  and boat  vinyl seats.  we literally have to crack the windows open a little in the vehicles when it's that hot.  luckily I will be hunting down in the Superstition mtn's and wilderness on Coues White tail hunt in Nov so the heat won't be a problem, rough ass country long shots are probable.  will be using a little Vanguard synthetic in 6.5 creedmoor  with first focal plane Nikon 4x16.

Putting in for spring Turkey on the north rim of the Grand Canyon,  hopefully I can get drawn, the plan is to take the wife up on a scenic  side by side drive around and look at all the northern points looking down on the canyon. getting older probably get a motel in Kanab Utah and short drive into AZ.  We have been up on the north Kaibab and strip  deer hunting several times always something interesting to see,  will stop in at Muley Crazy (the magazine) in Kanab and look at all the trophy deer  mounts, we try to stop in and visit  and get good info. 

[Dino82520]

On a side note, I recall reading an article where the author reportedly had some varmint loads that he carried in the glove box of his truck for a couple years. He reportedly stated that the first shot of that older ammo resulted in a stiff bolt opening.  The second round reportedly froze the bolt. He deduced that the cumulative vibration over a couple years resulted in causing the deterrent coatings to rub off the powder granules or the granules themselves to be ground into finer powder. Either way it supposedly caused high pressures.

Since reading that article I never leave ammo in any vehicle.

[Grouchy]

I don't normally leave a gun or ammo in my truck.  The waiting room at the doctor's office would be the single exception. 

[zoniezonie]

Dino82520 is absolutely correct ,  worst case scenario would be trying to polish loaded ammo in a vibratory tumbler or regular drum type tumbler very bad idea.  It's just not rifle ammo but pistol ammo with a much hotter powders in weaker guns.  I think maybe the best way to do that if you must carry ammo in a vehicle on a daily basis that is getting shaken around pretty hard would be to rotate the ammo on a regular basis, go out and shoot it and put in fresh ammo.

[Northern hunter]

Had it happen, but not hot vehicle.  Following recipe in Hornady book and took my 7mm to max load not realizing that 7828 is not temp stable. Weather warmed up and couldn't use it until the fall when it hit zero C again.

[BB340]

Had it happen, but not hot vehicle.  Following recipe in Hornady book and took my 7mm to max load not realizing that 7828 is not temp stable. Weather warmed up and couldn't use it until the fall when it hit zero C again.

I had same thing happen. I had a nice load for my .257 Wby mag that was at the max book load. Then when I was heading to Africa I dropped the charge by 2 grains just so as it wasn't a max load and I wouldn't have any hassles with it. It turned out Luckly that I did because we hit temperatures around 47 degrees for the whole trip and it caused my loads to become difficult to extract. One case blew a primer as the rifle had been sitting in the sun waiting for baboons to come and feed. I have stop using RE25 now due to it not being temp stable.