Impact of temperature and humidity on ammunition

[chrisintexas]

I want to know in what ways temperature and humidity impacts ammunition when you shoot it. Like say firing in Alaskan type cold and vs Arizona type dry heat.

 

[TXAZ]

Depends on the powder, google “reloading powder temperature sensitivity chart” and there is more specifics than you probably want.

 

[JohnKSa]

Here's a pretty good discussion of the topic.

https://thefiringline.com/forums/showthread.php?t=597035

 

[rock185]

chrisintexas, No doubt ammunition manufacturers have studied this, but I've not seen the information shared with the public. I have always stored ammunition under conditions that would comfortable to humans. A few years ago, I had the opportunity to chronograph some old Winchester .357 Magnum 125 grain ammunition that had been stored in a metal shed in Arizona for 20+ years. Temperatures outside the shed sometimes exceeded 100°, inside undoubtedly more. The ammo all fired 100%, but velocities were erratic, and velocities higher than expected with some. My informal and unscientific test suggested to me that the ammunition had deteriorated due to long term storage in temperatures ranging from lows in the teens in winter, to 100°+ in summer. I doubt humidity was a factor in our area, and suspect the heat as the major culprit in any deterioration of the ammo I tested. While 200 rounds I shot all reliably went Bang, I would not have wanted to carry it......ymmv

 

[chrisintexas]

Well I also wanted to know how temp and humidity will impact MV and ME when you fire them in Alaskan type cold and Arizona tyoe dry hot weather. Thanks so much.

 

[Aguila Blanca]

There's another factor to consider, too. Air temperature affects air density, and density affects both velocity and stabilization of the projectile. Read up on the history of the M16 rifle, the original 55-grain ammunition, and barrel twist.

 

[jmr40]

With most powders a rifles muzzle velocity will change somewhere between 1-3 fps for every 1 degree temperature changes. Humidity has no effect on muzzle velocity, but can have a minor effect downrange. As will altitude and shooting angles up or down. Most any decent ballistics program will have the option to input temperature, humidity, shooting angle, and altitude to be factored in.

If you're shooting at temps of -10 or colder your ammo could be more than 200 fps slower.

It swings both ways too. Most published numbers are at 70*. Here in GA the outside temperature could easily be 100*+ during August and if ammo is left inside a vehicle it could reach 140*. A load that is perfectly safe at 70* COULD be over pressure in extreme heat.

But there are some powders that are temperature resistant. They are still effected by temperature, but much less so. Closer to 1/2 fps for each 1 degree of temperature change. As a general rule I try to use those types of powders with any of my loads. That is part of the requirement for military spec ammo.

 

[JohnKSa]

Well I also wanted to know how temp and humidity will impact MV and ME when you fire them in Alaskan type cold and Arizona tyoe dry hot weather.

It depends. There is at least one manufacturer making ammunition which they claim will not change velocity based on temperature.

A chronograph is really the way to find out how ammunition is affected. But that's not always possible.

You will need to modify both the velocity and ballistic coefficient. Here are some formulas that should get you in the ballpark--but understand that they are only approximations.

SAAMI mandates that velocity measurements should be made at 70 degrees F. Therefore, for an approximation of velocity based on measured temperature you can subtract 2fps from the specified velocity for every degree Fahrenheit under 70 degrees or add 2fps from the specified velocity for every degree above 70 degrees.

Ballistic Coefficients are measured at 59 degrees F and 29.53mm of mercury. If you have a ballistic coefficient, you can adjust it using the following formula.

The correction factor for temperature is the current temperature in degrees Fahrenheit plus 459.4 divided by 518.4

Temperature Correction Factor = (Temperature in degrees F + 459.4)/518.4

The correction factor for barometric pressure is 29.53 divided by the current barometric pressure in mm of Hg

Pressure Correction Factor = 29.53/Current Barometric Pressure.

Corrected Ballistic Coefficient = Nominal Ballistic Coefficient x Temperature Correction Factor x Pressure Correction Factor


Here's an example.

The bullet you are using has a nominal ballistic coefficient of 0.50 and the U.S. ammunition manufacturer states that the muzzle velocity is 2700fps.

You will be shooting at 15 degrees F and 30.5mm Hg pressure.

First, correct the velocity. SAAMI velocity measurements are supposed to be made at 70 degrees F. 15 degrees F is 55 degrees below that and we assume a loss of 2fps per degree. 2 x 55 is 110 so we assume that the actual muzzle velocity at 15 degrees will be 2700fps - 110fps = 2590fps.

Calculate the temperature correction factor for 15 degrees F.

Temperature Correction Factor = (15 degrees F + 459.4)/518.4 = 0.9151

Calculate the pressure correction factor for 30.5mmHg.

Pressure Correction Factor = 29.53/30.5 = 0.9682

Now use the Correction Factors to adjust the Nominal Ballistic Coefficient.

Corrected Ballistic Coefficient = 0.5 x 0.9151 x 0.9682 = 0.443

So we went from a BC of 0.5 and a muzzle velocity of 2700fps to a BC of 0.443 and a muzzle velocity of 2590fps.

 

[Erno86]

Among other factors: The colder the temperature, the more dense the air; thus making the bullet travel more slowly through the air.

During the summer...I do not store my ammo at the range, where it is subject to the intense rays of the sun --- due to temperature differentials --- The same goes for my soon to be shot firearms...because, one side of the barrel heats up from the rays of the sun, thus affecting the point of impact on the target.

 

[FindANewSlant]

I don't know specific effects, but I know that you should store in cool, dark, dry places. I would imagine most ammo can be fired within spec in most climates, given they are sold in wildly varying climates.

 

[50 shooter]

Another issue with air density, humidity... is that you can actually see the bullet fly through the air. Until you see it, I tell people that it looks like you're watching the Matrix. In the scene where Neo is dodging the bullets and you can see them flying through the air.

I first noticed it when firing the .50 but now that I know what to look for, I've seen .30 cal bullets also. Helps to call shots for the shooter or if you're quick enough to get back on your scope, you can catch them.