Reloading - Inside and Out

The curtain of sideways snow currently visible from my loading room window set me thinking about the effects of weather on ammunition and rifle performance. I guess that we’ve all heard the phrase ‘colder equals slower, but that doesn’t really give us any science to work with. In fact, the words are potentially misleading, as a drop in temperature aligned with a reduction in air density may have the opposite effect, especially if we factor in such things as chamber and barrel temperature.

Missing the point

Before we attempt to quantify things, let’s consider a practical illustration of any change that may occur. Let’s take our long range, pest control or hunting rifle to the zero range with a box of our dedicated ammo. It’s a nice bright, warm day with low humidity. We select our desired zero distance and make the necessary adjustments. Being a bit pedantic I prefer to work from a closer start point and then progress to the desired point-blank range, noting the necessary clicks of adjustment along the way. However, in any event all this shooting has warmed the action of the rifle, whilst the box of ammo has been basking in the sun. Content with the mornings work, we clean the rifle and lock it away. A month or so later, the weather is wet, windy, cold and miserable but we’re committed to the shoot. A warming shot would simply warn the quarry of our presence. So, where is our first and hopefully, killing shot going to strike? Best guess is low, but by how much? What follows is NOT the result of personal research but collated from several credible published and validated sources.

 

Cooking the ammo

Let us start with the ammunition. All other things being equal, the colder the cartridge is at the time at which it is fired, the lower the muzzle velocity will be. I quote from the NRA of America Publication, American Hunter: “Temperature has a marked effect on both velocity and pressure. For IMR powders, each change of temperature of 1° Fahrenheit changes the muzzle velocity by 1.7 fps in the same direction. Thus, an increase in the temperature of the powder amounting to 20º F could be expected to increase the MV by 34 fps. It should be borne in mind that ammo exposed to the direct rays of the summer sun will reach a temperature much higher than that of the surrounding air. A temp within the ammunition of 130º F or even considerably more would not be unusual under such conditions. Even worse are the condition, within the baggage compartment of an automobile, especially one of dark colour.”

As a practical example, if you sight in your .243 Winchester before the deer season in 40° weather, then go prairie dog hunting in the 100° summer heat, where your cartridges are exposed to the sun (reaching 130° inside the case), expect an increase of around 136 fps. That’s around three inches at 300 yards! Extensive testing during the 1930’s at the U.S. Army Frankford Arsenal facility arrived at a similar range of data. It may seem relatively inconsequential but if the cartridge was loaded at the maximum safe SAAMI pressure then the increase in pressure could amount to another 5500 psi.

 

A chaffing disaster

The most detailed investigation into the effects of elevated temperature on small arms propellant involved double based Bullseye being used as the propellant in U.S.

Army M-796 Impulse Cartridges designed to dispense countermeasure flares (chaff). After a series of near catastrophic failures, the Army commissioned an investigation in 1978.

They discovered that long term high (over 85ºC) conditioning caused rapid and irreversible chemical degradation that seriously impacted upon the deflagration properties of the powder. Of more relevance to us was the fact that lower temperature conditioning also caused accelerated breakdown. Hercules (now ATK) quoted a maximum exposure temperature of 75ºC.

If we look at the effect of just air temperature on external ballistics, then the variation per degree is minimal. Taking a common hunting round, the .270 Winchester with a 130-grainTTSX bullet. The Barnes Vor-Tx factory load quotes a muzzle velocity of 3060 fps. Using the Hornady Ballistics Calculator to set the altitude, barometric pressure, and humidity to STP we can observe the variation in drop as follows:

Distance: 200 300 400 500 (yards)
100 F° 2.8” 10.7” 24.6” 45.4”
80 F° 2.9” 10.9” 25” 46.3”
30 F° 3” 11.2” 26” 48.6”
0 F° 3” 11.5” 26.7” 50.3”

In isolation, the sub one-inch variation at the practical, ‘real world’, 300-yard range of this cartridge is too small to be of concern. However, we may well need to read this figure in conjunction with the internal ballistic variation created by the change in ammo temperature. That DOES make a difference.

 

Seeing things

The next sunny day atmospheric effect that plays games with our accuracy is mirage. I’ll quote a popular dictionary: “an optical phenomenon, especially in the desert or at sea, by which the image of some object appears displaced above, below, or to one side of its true position because of spatial variations of the index of refraction of air.”

A crude way of demonstrating it is to take a glass full of water and put a spoon in it. Note how the spoon handle appears to bend where it enters the water. This is caused by the difference in the refractive indexes of air and water. On the range, the variation of air temperature between you and the target causes a change in the refractive index of the air along your sight path. This distorts the light, causing the image of the target to be in a different place than the actual target. Bench resters are familiar with the vision of a target dancing around their static cross-hairs. Indeed, the use of a scope can complicate matters, as the intentionally limited depth of field does not always reveal the true effect of the distortion. Furthermore, mirage can vary significantly with just small changes in elevation above the ground. A prone shooter may experience significant distortion, whilst his neighbour using crossed sticks may be unaware of the effect.

 

And another!

A further complication for the shooter in the foxhole or flat on the ground is called ground effect, sometimes described as ground signature (although this more specifically relates to the behaviour of muzzle brakes). This is where the conical blast of gas ejected from the muzzle is distorted by the ground, or any other near obstruction, just ahead of the muzzle. If the fired projectile was ahead of this cone, then the distortion would have little effect.

However, at the muzzle and for some distance beyond, the velocity of the gas cone exceeds that of the projectile and is also continuing to provide thrust to the bullet, so can also affect trajectory and therefore point of impact.