Some years ago my buddy Dave and I traveled from Illinois to hunt antelope on a high plateau in Southern Utah — almost 10,000 feet high. Noon on opening day a big coyote appeared around 300 yards off. Dave got steady on his bipod and took a crack at it with his super-accurate McMillan .300 Win. Mag. — and shot several inches over its back. He missed high again as it ran, then dropped it with a backbone hold when it stopped at 385 yards.
After that, we took some time to verify his point of impact as distances stretched. Sure enough, although his 100-yard groups still predictably impacted 1.5 inches high, at 200 yards — where he was theoretically zeroed — impact was a little high, and at 300 and 400 yards his bullets didn't drop nearly as much as his chart showed. We'd forgotten to factor in the effect of elevation, temperature, and the arid climate.
You'd think that a fast, hard-hitting cartridge would shrug off environmental influences, but not so. Sure, such rounds minimize disparity compared to more pedestrian calibers, but disparity there will be.
The Air Density Equation
First, it's important to understand why there are differences. Without getting really scientific, it mostly boils down to air density. Denser air applies more friction to a projectile, slowing it down more quickly than less-dense air would. Three factors cause air density: low elevations, low temperatures, and high humidity.
The effect of low elevation makes sense — heavy air settles, so the lower you are the denser it gets — but the low temperature factor seems counterintuitive. It would stand to reason that cold, brisk air should apply less friction than hot, oppressive-feeling air.
But cold air is, in fact, denser. Have you ever opened your freezer and felt the cold air flow downward? This is what happens in the atmosphere; it separates from the warmer air and the molecules pack together.
High humidity has less of an effect than elevation or temperature, but it does have an effect. The lower the moisture content in the air, the less dense it is, thus the less drag it applies to a bullet.
Let's take a .300 magnum like Dave was shooting. It spits a common 150-grain flat base hunting bullet, such as a Remington Core-Lokt (BC .294), out of the muzzle at about 3,290 fps. We calculated it on JBM Ballistics with parameters set to represent an Eastern shooting range at 400 feet elevation and estimated environmental conditions typical of pre-hunting-season months: 90 degrees Fahrenheit, 80 percent humidity, and, for simplicity's sake, a set barometric pressure of 29.92. This is opposed to conditions representative of a high-country elk or mule deer at 10,000 feet elevation and October-like environmental conditions: 30 degrees Fahrenheit, 20 percent humidity, and the same barometric pressure.
With all that data we see the following disparity in 500-yard projectile performance:
Note that even with a 60-degree decrease in temperature, which greatly offsets the effect of increased altitude, there's an 8-inch difference in bullet drop — plenty to cause a miss on a deer-size animal.
Let's now assume that you're hunting during late "Indian Summer" conditions and that temps are hitting 60 degrees by midmorning. Look at the difference now:
There's an incredible 17 inches of difference in drop and almost 600 fps difference in velocity. Double the velocity difference between a .30-06 and a .300 Win. Mag. shooting equal bullet weights.
Assuming you know your drop in minutes of angle (MOA), an old rule of thumb is to add 0.5 to 1.0 MOA per 5,000 feet of elevation gain: "elevation up, bullet up." And to compensate 0.5 to 1.0 MOA per 20 degrees (Fahrenheit) of temperature change: "cold down, warm up." It's a useful guideline as long as you recognize its limitations.
The Bullet Weight Factor
The numbers above also illustrate that choosing a quality, streamlined bullet for Western shooting is key. Long shots are far more common than in most other parts of the country. A slow caliber stoked with high-BC bullets will display less disparity than a fast magnum with low-BC projectiles.
Using a heavy-for-caliber, aerodynamic (high BC) bullet, such as Swift's 180-grain Scirocco II (BC .520), will minimize this difference. Under the same conditions as the second chart, it performs as follows:
The charts above clearly show that bullet choice has an enormous effect. My favorites are any of the polymer-tipped, bonded, boat-tailed premium bullets common on today's market, like Nosler's AccuBond and AccuBond Long Range, Swift's Scirocco II, Hornady's InterBond, and Barnes's homogeneous LRX (Long Range X). With such a bullet the disparity between your home-turf, low-elevation range and high-altitude mountain conditions will be negligible enough that you can ignore it — unless you shoot past 400 yards.
The Long(er) Range Factor
If you're one of today's crop of hunters that intends to shoot at game past 400 yards, you'll want to research your intended hunting area and build multiple drop charts for the correct altitude, bracketing various anticipated temperatures. As much as I love ballistic drop compensating reticles, such as Zeiss' Rapid-Z 800, here's where dialing turrets has a tangible advantage in precision: You can use those prebuilt charts to dial to fit current environmental conditions.
Finally, if you're hunting the West and preparing to take shots beyond your Eastern comfort zone, you've probably chosen a fairly zesty cartridge with good muzzle velocities. Don't cripple your potential by sighting in at 100 yards. I don't care if your daddy back through his granddad's daddy all did — sighting in any scoped, high-power rifle at less than 200 yards is a piteous waste of a perfectly good flat trajectory. If you're shooting a very fast cartridge, such as the .257 Weatherby or the hot new 26 Nosler, consider zeroing at 250 or 300 yards.
Zeroed at 200 yards, you can hold 6 to 8 inches high at 300 yards and connect with most rifle cartridges. Zeroed at 100 yards, you'll have to hold close to a foot and a half high at 300 yards — a much less precise method — and forget about shooting at 400 yards.
Not comfortable shooting at 400 yards? Get comfortable. Learn to shoot prone — correctly — with a bipod or daypack and polish your technique until you can keep all your shots on a paper plate at a quarter mile. If you're limited to a 100-yard range at home, practice shooting tiny groups — if you can consistently put all your shots into 1.5-inch groups at 100 yards from your prone field position, you'll be just fine at 400 yards as long as you know your drop. Past 400? You'd better know your Western ballistics.