For every 20 degrees of temperature change, adjust one MOA to compensate for the ballistics change." Everyone who has ever heard that in a sniper school, please raise your hand.
It must be true, because it's printed in the U.S. Army sniper manual and spoken in nearly every basic sniper school in the country. Those of you who know me know that I'll tell you it's B.S. Actually, it is technically true, but only at such far distances that they exceed any recorded law enforcement sniper shot ever made. This rule is dangerous because it tells police snipers to start changing their sights when it's not necessary. Doing so leads you away from the correct firing solution. The problem is that it has been taught for so long now that it has nearly become sniping gospel.
This rule continues to be repeated and accepted despite the fact that it is so easily proved wrong by testing in the field under a variety of temperatures. As snipers, isn't that what we are supposed to do? Shoot under a variety of circumstances and carefully note the results? Bad information can lead to bad results.
Sniper instructor John Simpson has a great saying: "If you can't show me the math, then it's just your opinion." I can show you the math. After that, it's up to you to go to the field and verify with your rifle what I am about to explain.
There are two factors to temperature effect: internal ballistics and external ballistics. Internal ballistics is what happens within the gun. So the question is, how does the temperature of the propellant affect its rate of combustion and thus projectile velocity? Answer: Typically, higher temperatures cause the powder to burn faster, generating more pressure and velocity. External ballistics is what happens to the projectile during its flight upon exiting the muzzle to the target. Which takes us to our second question: How does the temperature of the air affect the flight of the bullet? Answer: Cold air is denser than warm air. Cold air thus provides more drag on the bullet and causes it to lose velocity more quickly. As the bullet slows, it drops more quickly; that is a bullet's vertical drop relative to the distance traveled.
The problem we face is that while the manual is technically correct at long distances, law enforcement sniping usually takes place at between zero and 200 yards. Even most military shots are made within 500 yards, so those wearing a military uniform should pay attention as well.
THE NEW RULE
I will make a bold statement here that makes everything pretty simple to calculate. Here's the new rule: For sniping to 500 yards, ignore temperature effect. With that gauntlet thrown down, here's the math.
INTERNAL BALLISTIC EFFECT
With a .308 Win. cartridge, you can generally assume that for every degree of temperature change, you can expect approximately one to 1½ fps in velocity change. I can show the test results behind the generalization if anyone wants to see them. Let's look at what effect we might experience. To keep the math simple, let's work with the 20-degree change that the sniper manual mentions. That easily translates to a velocity effect of 20 to 30 fps.
You should note that there are modern, temperature-insensitive powders available that essentially eliminate even that change. My company, Black Hills Ammunition, uses these new propellants in our .308 Win. and 7.62x51 NATO 175-grain loads, and we are moving it to other .308-caliber loads, but for the purposes of this article, let's assume that the 20 to 30 fps variation still exists.
Decreasing the starting velocity from 2,650 fps to 2,620 fps at sea level with a 168-grain BTHP match bullet results in an added drop of 1.7 inches at 500 yards. Remember this, because we will now check the external ballistic effect, then combine the two effects to see what the total effect actually is.
EXTERNAL BALLISTIC EFFECT
The difference in air density resulting in a change from 80 to 60 degrees amounts to a bullet-drop difference of .9 inch at 500 yards. The combined internal and external ballistic effect of a 20-degree temperature change is 2.6 inches at 500 yards, approximately half MOA.
The "one MOA per 20 degrees of temperature" rule does not become true until 700 yards, even when combining the internal and external ballistic effect. The variation may be half MOA at 500 yards, but it's less than that and absolutely insignificant at the more common law enforcement engagement distances of 200 yards or less. The effect is less than the change in point of impact that you can induce by changing body position or failing to adjust your parallax. I submit to you that it is so insignificant, you should ignore it altogether.
SPEED AND PRECISION
I realize that our business as snipers is to be precise, but it's also to be fast. If you spend time diddling with insignificant variables while you should be concentrating on the more important factors, you'll lose the race. The important factors after evaluating and deciding whether to even use deadly force are range to the target, wind effect and required leads. If you spend time calculating, then apply an inappropriate rule, the result is worse than the wasted time. It can mean a miss.
Even an extreme swing of 80 degrees will only result in a maximum of a half-MOA point of impact change at 200 yards compared with the four-MOA shift that would be predicted by the often-repeated rule of "one MOA adjustment for every 20 degrees of temperature change." If you apply the "20 degree" rule to a 200-yard shot in this circumstance, the result is a miss by seven inches.
That's the math. Want real-world results? I am a law enforcement sniper in South Dakota. I have shot in extremes from 30 degrees below zero to 110. I average around 1,600 rounds per year recorded through my .308-caliber sniper rifle. I have steel ranges set up and waiting for me any time I want to go shoot. I regularly train to 600 yards, in all temperatures. My observations in the field verify the computer solution. Out to 500 yards, I ignore the temperature. Take the time to verify temperature effect for yourself.
Many times we experience a small variation in our zero from the last time we shot, and as reasoning snipers we try to explain it. I do it, too. We have an obligation to ourselves to ask those questions. I can tell you, however, that when it happens at ranges of less than 500 yards, the math won't back up the conclusion that temperature-induced internal or external ballistic variations are significant factors. Keep looking in another area. Hint: It won't be humidity, altitude or barometric pressure. These are variables best covered in another article.