Does leaving ammo in a hot vehicle decrease accuracy?

Would seem so if this rule still applies. From a Google search. Gunpowder heat sensitivity.
“The old rule was to expect an adjustment of 2 fps per degree Fahrenheit, up or down from an ambient temperature of 68˚F, and my experiences had pretty much proved that to be true.”

Increase/decrease in velocity equals bullet drop or rise.

Note: I didn’t read the entire article. The author was speaking in terms of guns powders of yesterday years. Perhaps new Gun powders are more reliable in terms of pressure curves created by ambient temperature.

In my experience, no. My reloading bench is in my garage. Living in Phoenix, on the really hot days when it's 115 degrees or more outside my garage can get to around 150 degrees. Even during the "cooler" days of summer that garage is 120+ every day from June to October. Ive left loaded handloads and factory match ammo in that garage over the summer that still shot sub 1/2 MOA.

Now shooting when the ammunition is still warm/hot will change the velocity it shoots at and could potentially change accuracy. If you watch the interview with, I believe, Corporal Furlong (dont know the spelling) who held the longest sniper shot record for a while..... He talks about leaving the ammo out in the sun to get it warmer to increase muzzle velocity for the extreme range shots.

Accuracy...? Probably not. Can it change the characteristics of the ammo? Probably, but that depends.

I shoot a lot in the NV desert... in the winter it's around 60F, in the summer ambient temps can be in the hundred-teens... and who knows how hot ammo in direct sunlight will get? The hotter ammo shows up as higher velocity, all else being equal. Changes in velocity = changes in ballistics, and, remotely, changes in pressures. Factory ammos might not be such a big deal, but reloads with temperature sensitive powders might be.

I've actually pondered this question for a while. For some time, I had a basic load of ammo I was carrying in the truck with me... a small stash of 9mm and 5.56mm. Temperature extremes are not necessarily kind to powders, but I was more worried about the long-term effects vibration and vehicular motion might have on the powder itself, and particularly any sort of coating rifle powder might have. Granted, it was surplus M193, very likely with ball powder, but hunting ammos might very well have extruded powders...

I have since stopped carrying ammos in the truck with me, although I never found a direct answer to my question. I'll be curious to see the other responses posted here...

You’ll never know unless you chronograph rounds under all the extremes in temperature likely encountered.

As to powder burning characteristics under constant storage subjected to transportation movements how does the military cope with that. Helicopter or tank transport couldn’t get any more shaky than that.

All of our ammunition and explosives in afg were stored in shipping containers that were not climate controlled- never had issues with any of our stuff not working as advertised- including ammo for precision sniper systems. But a live round in the chamber of a rifle on a very hot day will "supercharge" (that's what we called it) and increase velocity- making that round hit high. You can actually feel the difference in the recoil. So, we made sure our students in the summer "vented" (cracked) the chambers of their rifles on the firing line, and kept their boxes/magazines of ammo out of the sun.

Some powder is very stable across a wide spectrum of temps. This is a huge part of military specs when they order ammo. They need it to be consistent across a wide range of temperatures from the Arctic to the Desert.

But MOST gun powder will see somewhere between 1-3 fps velocity change for every 1 degree temperature changes. It can get 150 degrees or more inside of a vehicle here in the south in the summer. A round meant to give you 2800 fps at 70 degrees ( this is the normal temperature for testing) could easily be 200-300 fps slower if shot at -20; or 150-250 fps faster if shot at 150 degrees. At that temperature you could have an overloaded round and a dangerous situation.

There are several powders out there that are intended to be temperature resistant. They are still effected by temperature, just not as much. Closer to 1/2 fps per 1 degree temperature. That would mean that in the example above our 2800 fps round at 70 degrees would now only be about 50 fps slower at -20 degrees and about 40 fps faster at 150 degrees.

Long range military snipers factor in for the temperature when calculating hold over at extreme ranges. Temperature also effects the bullets trajectory through the air. But for most shooters the 40-50 fps swings aren't enough to matter. But 200-300 fps can make a difference. This is why I prefer to use temperature resistant powders in my hunting loads. Loads only used at the range for target practice I don't worry as much about.

When buying factory loads I'd assume they aren't using temperature resistant powders. And I'd be concerned about using loads that had been left at extreme temperatures while they are still hot. From what I've read there isn't any long term issue. Once it cools down to a more normal temp it should be fine.

I was more worried about the long-term effects vibration and vehicular motion might have on the powder itself,

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Reading an article some time ago where an African guide decided to pull some bullets from a box of ammo that had been left bouncing around in his truck for years. The powder was supposed to be stick powder, but had vibrated into a very find powder. That has to change the burn characteristics. The guy didn't do any further testing opting instead to just trash that box of ammo.

This was a big campfire conversation we had around the campfire one year in South Dakota. We were a pretty diverse group of guys that met up shooting Prairie Dogs. One older feller had all kinds of gizmos and gadgets and some nice fancy oddball caliber rifles. He kept all of his ammo in styrofoam containers and swore about keeping his ammo from getting to hot. I had my ammo all in 50 round plastic boxes in a black range bag. One bag for each rifle. Another guy was shooting Black Hills in the factory 20 round boxes. The old guy said we would all get better results by storing our ammo like he did. The next day out in the field we all shot well and the old feller pulled out his infra red temp gun and once out on the bench all the ammo and rifle barrels were pretty much all the same temp.
Bottom line: we all had fun shooting and challenging each other on the longer shots. Our host, a local rancher shooting a boat paddle Ruger 22-250 and a Tasco 3x9 loading max load IMR3031 and Hornady 55g Vmax could hit anything he shot at. Kept his loads in a Folgers coffee can.

Our local sheriffs office swaps out ammo annually in handguns as well as trunk rifles after several deputies suffered rifle malfunctions caused by bullets “rattling loose” after a couple of seasons in their trunks. Our office takes the same mentality, thigh we didn’t have any loose-bullet issues in trunk guns ourselves.

You could probably get away with keeping a box under your seat, but it may be better to just keep the box in the safe by your rifle so you grab them together when you load up to go hunt.

Stay safe.

Powder temp is one of the factors that are used in the calculations for field artillery.

So is rotation of the earth, so how much it affects accuracy in a little gun im not sure.

Considering that the OP isn't discussing leaving handloaded ammo but factory ammo in the vehicle, and it's hunting rounds not precision bench rest ammo... I'd say leaving it in a vehicle for an extended period isnt gonna be an issue.

If velocities are 50-100fps off one direction or another, chances are game animals aren't gonna notice a very slight change in POI. Unless the OP is shooting exceptionally long distances at his quarry.

Like the stated above.

Technically yes. I have had rounds left in my truck at °165(forgot they were sitting on the dashboard fer a few hours) and didn't group well until I cooled em off in the shade.
They were speeding just off the node I loaded them for.
Came back to the original group size after they cooled off.

So yes it can. But it'll be a extreme circumstance.
Mostly it will be a non issue.

whughett said:

As to powder burning characteristics under constant storage subjected to transportation movements how does the military cope with that.

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jmr40 said:

Reading an article some time ago where an African guide decided to pull some bullets from a box of ammo that had been left bouncing around in his truck for years. The powder was supposed to be stick powder, but had vibrated into a very find powder. That has to change the burn characteristics. The guy didn't do any further testing opting instead to just trash that box of ammo.

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Riomouse911 said:

Our local sheriffs office swaps out ammo annually in handguns as well as trunk rifles after several deputies suffered rifle malfunctions caused by bullets “rattling loose” after a couple of seasons in their trunks.

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There are really 2 issues to the OP's question. Everyone knows temperature extremes can change the immediacy of ballistics... I've seen it myself shooting in the desert. It's also known that extreme heat will cause powder to break down faster over time. Conversely, extreme cold can affect ammos, too.

Military cartridges use ball powders (although that is likely not true for their precision ammos.) Contrary to belief, a case of ammo is not riding around in a truck for years and years... it is stored (perhaps long-term, perhaps not,) issued, and likely used in fairly short order.

My biggest concern would be with ammo using extruded powder, and the deterioration of the powder coatings, and even the powder structure itself... riding around in a vehicle for years at a time.

Charlie98 said:

There are really 2 issues to the OP's question. Everyone knows temperature extremes can change the immediacy of ballistics... I've seen it myself shooting in the desert. It's also known that extreme heat will cause powder to break down faster over time. Conversely, extreme cold can affect ammos, too.

Military cartridges use ball powders (although that is likely not true for their precision ammos.) Contrary to belief, a case of ammo is not riding around in a truck for years and years... it is stored (perhaps long-term, perhaps not,) issued, and likely used in fairly short order.

My biggest concern would be with ammo using extruded powder, and the deterioration of the powder coatings, and even the powder structure itself... riding around in a vehicle for years at a time.

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Powder deteriorating from temp extremes is certainly a concern. I have no idea how long it would take, but the extremes in my local area can run from the teens to well north of a hundred and forty degrees in a vehicle. And this sometimes occurs in a matter of days depending on if the unit is assigned to the mountains and then down in the desert. Long guns ride secured in steel locking racks that offer no padding like a soft case, so every rattle and jolt is transferred straight to the rifle/magazines if roads are rough or unpaved.

The “rattling loose” did happen to a few guns during a highly publicized manhunt down here a few years back. Several 223 rounds riding for far too long in magazines had bullets come loose. These issues weren’t discovered or noticed until the rifles were deployed during the chase and shootout.

It may gave been from poor crimping and QC at the ammo plant, it may have been from abuse by whoever used that car/rifle combo over the years, who knows. All I know is the range/training staff was so concerned that their office instituted an ammo rotation policy so rounds aren’t in guns more than one year for sidearms and two for long guns.

Stay safe.

First in, first out. Shoot your stowed ammo as you practice with the stowed weapon and replace it with fresh.

It’s been documented that some powders may also mechanically degrade through the jostling of sustained, long term vehicle transport. The powder impinges upon itself and the cartridge case, milling itself over time - which changes its primary combustion properties.

Many folks claim double-base powders will sweat nitroglycerin if exposed to extreme and repeated thermal cycles.

I shoot too much, including rotating first in, first out my cached ammo, to ever have direct experience with age degradation in my vehicle.

I don't think so. Temps here can range from just over a 100 to -40, although I normally don't shoot much below zero. The thing is that all the ammo is at the same temp so it can change velocity somewhat it will be the same for all the ammo you have with you so groups would not vary much if any. POI may vary slightly but not enough to matter for most shooting.

I've never in my life forgotten my ammo when going afield. I have however, grabbed the wrong ammo (20ga instead of 12ga) and have on more than one occasion rendered a semi-auto .22 into a single shot due to lost/misplaced magazine.

So, how hot is it during your hunting seasons?