A hundred years ago, on an Alberta whitetail hunt, I endured a really nasty cold snap. You'd think the deer movement would be crazy, but with temperatures plummeting far below zero the animals must have been as shell-shocked as I was. I never saw a deer that first day, but when we got back to camp that night I checked my rifle, and the firing mechanism was frozen stiff — no way I could get the hammer to fall. I thawed the bolt out, stripped it down and degreased it (again), thankful I hadn't had a chance at a great buck.
Two hundred years ago I took a Savage 99 in .308 to Africa along with some really good handloads. In those days I was pretty young — bulletproof and invincible like all youngsters. I was inclined to bypass all that silly working up stuff and go straight to the listed "maximum" and did my range work on 60-degree May days.
Over there we caught an unusually hot day, well into the 90s, perhaps more. The rifle's first shot in Africa was at a blesbok, "hit and away," as Ruark wrote. That first shot was also the rifle's last shot in Africa. The fired case was stuck in the chamber and, lacking the camming power of a bolt, the lever was unable to open the action.
For the short term, one of the trackers ran back to the truck to fetch another rifle, and I finished a poorly started job. For the long term, well, that rifle went home with the action closed on a fired cartridge (today there would be no way to get a rifle in such a condition through security) and went back to Savage exactly that way. They fixed it, God knows how.
That's cold and heat. Altitude and humidity are less dangerous in that they are unlikely to cause stoppages, but they do have insidious effect. In fact, any of these factors under extreme conditions will have an effect on your rifle and ammunition, and while you can't always do anything about it, it's important to know what's happening.
I hate cold, and I'm not very good in it. But some hunts have to be conducted in cold weather, and a lot of the hunting we all do — such as North American deer and elk — is generally best when the mercury drops. So deal with it.
If you're headed into serious cold the most important thing to do is degrease all moving parts of your rifle. How crucial this is depends on how cold it is and what lubricants you've used.
Just freezing shouldn't be a problem, and you're probably okay well down in the 20s. Down in the teens, better safe than sorry — but if the mercury plummets toward zero, most lubricants will get sluggish, and you will encounter a slow hammer fall at best — or total lack of function at worst.
Bolt actions are the easiest to degrease and the most likely to continue to function without lubricant. With a bolt gun, remove the bolt and strip it down if you can. If you can't, soak it repeatedly with a degreasing solvent. There are plenty of household or camp products that will do the trick: white gasoline, rubbing alcohol and nail polish remover are good ones.
For other action types, well, it gets tricky. An AR is easy because you can remove all the moving parts and soak them down, but other semiautomatics, pumps, lever actions and single-shots are more difficult. The best course is probably to remove the stock to avoid damage to the finish and soak the action. Some actions can't be guaranteed to work with liquid lubricants, but for the short term you can replace liquids with a bit of dry graphite.
Extreme cold often means snow, so put a strip of tape over the end of your muzzle. I've forgotten to do this a bunch of times, and I usually wind up going under a snowy tree and then spending valuable hunting time digging, blowing and sucking snow out of the muzzle (after unloading the gun, of course). As I and others have written before, a strip of tape over the muzzle has no effect on point of impact.
Depending on the temperature and the distance of your shot, you may need to worry about the effect of temperature on your cartridge's trajectory. The basic rule is: Colder equals more drop. The problem here is that this is a very inexact science because some propellants are more stable across a greater temperature range than others.
There is a rule of thumb. For every 20 degrees difference (and we deal in Fahrenheit here) from the temperature at which you zeroed, you can expect a drop of 0.5 to 1.0 m.o.a. So if you zeroed at 70 degrees and headed to the Arctic for muskox, caribou or polar bear and it was minus 30 when you arrived, you have a difference of 100 degrees. This should mean that your point of impact will be 2.5 to 5.0 m.o.a. lower. This is too great a variance to help you if you need to make a 300-yard shot, so the best plan is always to check your rifle on site, with the climatic conditions you will be hunting under. To hedge your bet, if you are going from warm to cold, you might consider sighting in a bit higher than you normally would.
Heat has the exact opposite effect, with the same rule of thumb: 0.5 to 1.0 m.o.a. for every 20 degrees warmer than your sight-in temperature. Just remember that rules of thumb are simply guidelines; I've never found any consistency with climatic influences on ballistics. However, it is absolutely true that when ambient temperature increases propellant powder gets "hotter" and produces more velocity, which should raise your point of impact.
There's no substitute for checking your rifle when you get to your hunting location, and if I'm going from temperate or cool to very hot, I usually sight-in dead on and plan to adjust on the ground as needed.
In extreme cold we're worried about the rifle functioning. In extreme heat there's another problem. I said earlier that as temperatures rise propellants get hotter, but what I really mean is pressures increase. I'm sure that's what happened with that Savage 99.
A century ago, when the Brits were loading volatile cordite propellant, they actually offered "tropical loads" that were a bit under-loaded for the English climate but came up to full velocity at the higher temperatures of Africa and India.
If you're going from cooler to warmer, stay away from maximum loads. And don't let your ammunition get overheated. Provided you aren't shooting red-line handloads, ambient temperature should not be a problem, even if it's over 100 degrees.
Metal such as brass heats in the sun, so keep your ammunition in the shade. Covered ammo pouches are better than open belts.
And it doesn't have to be over 100 outside for temperature to affect ammo. As I write this, it's not even above freezing, and just for fun I put some exposed cartridges on my dashboard, facing the sun. In a half-hour they were almost too hot to touch. It's something to keep in mind.
This is a chronic problem for me because I live near sea level, but I love mountain hunting. This often entails longer shooting, so small differences in your assumed zero can really mess you up.
Some time back I got one of Leupold's new VX-6 scopes with a custom turret calibrated to a favorite load in .300 Wby. Mag. Of course I had to tell them at what altitude I would be shooting. Historically, I've taken game from 500 feet below sea level in the Danakil Depression to almost 18,000 feet in China. I told them 8,000 feet, which is probably a reasonable average for the altitude at which I'm most likely to use that cartridge.
This is at best a compromise, and just as with temperature, there's a rule of thumb for elevation: For every 5,000 feet of elevation difference between your sight-in elevation and the elevation at which you are hunting, you can expect (you guessed it) 0.5 to 1.0 m.o.a. difference.
At higher altitude the air thins, causing less resistance, so when you go up in elevation the strike of the bullet goes up. When you go down to "heavier" air, your strike goes down. I have not found this rule to be especially consistent, so when you get to altitude check your rifle.
Since I shoot at nearly sea level I usually sight-in at home to be dead-on or just a bit high at 100 yards, and then when I get to altitude I'll be pretty close to the zero I like, which is about 2.5 inches high at 100 yards.
So far everything we've talked about should be intuitive: Cold, down; heat, up; higher altitude, up; lower altitude, down. Humidity is the exact opposite of what seems obvious. Moist air is less dense than dry air, so if you go from a desert climate to an area with high humidity you can actually expect your point of impact to rise. The rule of thumb: You can expect about a 0.5 m.o.a. change for every 20 percent change in humidity. And, no, as far as anyone knows, direct precipitation does not impact the flight of a bullet — but snow or rain sure can make it hard to aim.
The bottom line is this: At our home ranges we spend lots of time getting our zeros exactly perfect, and then when we arrive at our hunting sites we generally do a perfunctory check. The reality is we probably won't see a difference of just 0.5 m.o.a, but the size of game animals' vital zones are usually large enough that we can get away with it.
For instance, in Africa there can be extreme climatic changes, but if we're talking a Cape buffalo at 100 yards even the most extreme difference won't matter. On the other hand, a Thomson gazelle out on the short-grass savanna at 400 yards is a much different story.
I've gone over the basic rules of thumb, which will help you know what you can expect, but there is simply no substitute for checking your zero on the ground.
The real rules are: There are no precise rules. So take the time to adjust your zero when you get to your destination. Do it right, take whatever time is required, and get it right.