It was known for a while that cold ammunition does not perform as well as ammunition at a warmer temperature. The higher the temperature of the ammunition, the higher the velocity of the bullet. Of course, the increase in velocity is also accompanied by an increase in the chamber pressure. This is caused because the powder in the ammunition burns at a faster rate when it is warm and at a slower rate when it is cold.
This phenomenon affects how ammunition should be stored. For instance, in a National match in 1930, it was found that ammunition that was exposed to direct sun rays for several hours before the match, was generating excessive pressure, causing some firearms to malfunction. The problem got so bad that the organizers had to stop the match and replace the ammunition with others stored at cooler temperatures.
However, ammunition stored at colder temperatures will ignite slower and not generate as much pressure and velocity, which could cause the bullet to reduce its range. Therefore, when people publish data about velocities and pressures generated by different ammunition types, this data is always measured at a certain standard temperature (in the United States, this is generally 59 degrees fahrenheit)
Modern ammunition is not as susceptible to the effect of temperature swings, but it still happens. So what is the amount of loss or gain due to variations in temperature, the reader asks?
Several different authorities attempted to answer this question in the 1930s. According to studies conducted by the Frankford Arsenal (located in Northeast Philadelphia, Pennsylvania) and Burnside Laboratory (located in Carney's Point, New Jersey), the ammunition they tested increased in velocity on average by about 1.7 feet per second for every one degree fahrenheit rise in temperature for ammunition loaded with IMR type smokeless powders. What this meant is that ammunition heated from (say) 70 degrees fahrenheit to (say) 130 degrees fahrenheit would experience an increase in velocity of about 102 feet/sec.
Correlation between ammunition temperatures and velocities for three different types of ammunition. Click on image to enlarge
This test was conducted by the US Ordinance Department and the image is now in the public domain.
Of course, the composition of the ammunition plays a large part in this and different ammunition types react to temperature in different ways. The US Ordinance Department also conducted its own tests in the 1930s and the results are shown in the graph above. As you can see from the above graph, Dupont's 1489 powder generates about 51000 PSI (pounds per square-inch) of pressure at 70 degrees fahrenheit, but generates about 56500 PSI of pressure at 140 degrees fahrenheit. This is an increase of about 5500 PSI (or 11% increase) and some firearms may not be able to handle the excess pressure generated.
In case you're wondering why 70 degrees fahrenheit appears in the graphs and notes above, it is because that was the "standard temperature" that measurements were taken against in the 1930s. These days, the standard temperature in the US is considered to be 59 degrees fahrenheit
The following table is based on U.S. Army tests on .308 Winchester ammunition, which is the commercial variant of the 7.62x51 mm NATO standard ammunition.
|Bullet Drop at
|-10||2400 feet/sec||-109 inches|
|+25||2500 feet/sec||-100 inches|
|+59||2600 feet/sec||-91 inches|
|+100||2700 feet/sec||-84 inches|
Bear in mind that air is a good insulator, whereas brass is a good conductor of heat. Therefore, ammunition that is exposed to direct sunlight may reach higher temperatures than the surrounding air and 130-140 degrees fahrenheit is not an unusual temperature for ammunition to reach. The excessive pressures may cause some firearms to malfunction and can also affect the accuracy of the weapon.
Consider that a hunting rifle has been zeroed in at a temperature of about 80 degrees fahrenheit (which is the average temperature on a typical day where the author lives in Southern California). Now let's say the person goes hunting with that rifle in Alaska, where the average temperature in early morning is typically about 0 degrees fahrenheit. That is a 80 degree variation in temperature, which can cause the velocity of the bullets to drop by a significant amount. This means the sights will need to be readjusted because of the drop in velocity and chamber pressures.
In the late 1800s, the British author and firearms manufacturer, W.W. Greener, came up with a pretty convenient approximate formula to account for changes in temperature. His formula is based on a standard temperature of 60 degrees fahrenheit (rather than the modern ballistic standard of 59 degrees F) and is as follows:
Range adjustment for temperature = (degrees +/- from 60 degrees fahrenheit) * (target distance in hundreds of yards) / 10
So if the temperature is above 60 degrees, we should subtract this distance from the actual range and if the temperature is below 60 degrees, we should add this distance to the actual range. For example, say the temperature is 100 degrees fahrenheit and the actual distance to the target is 800 yards. Therefore, we calculate:
Range adjustment for temperature = 40 * 8 / 10
which works out to 32 yards. What this means is that though the actual distance to the target is 800 yards, we should treat it as being at (800 - 32) = 768 yards and adjust the sights accordingly.