October 19, 2021
Photo of the new ZEISS LRP 318-50 first focal plan (FFP). A bullet that is launched at a higher altitude can fly slightly further (in thinner air) for each increment of downward movement. The bullet behaves effectively as if it had a higher ballistic coefficient. It’s hunting season and a good friend will be heading to Colorado next week to hunt elk. He recently zeroed in his rifle in California at a range of just a few hundred feet Above Mean Sea Level. He wondered if Colorado’s higher altitude could affect his ballistics. The answer is yes. The good news is that you can use free ballistics calculators to create reliable drop charts for different shooting locations, high and low. It is now clear that altitude has an effect on bullet flight. The simple answer is that at higher altitudes the air is thinner, which means there is less drag on a bullet. This means that bullet drop is lower at all flight distances from the muzzle. Practically, the force of gravity on the earth’s surface is constant. However, the bullet’s downward acceleration does not change. A bullet launched at higher altitudes is able fly slightly further (in thinner air) for each increment of downward movement. The bullet behaves at higher altitudes as if its ballistic coefficient is higher. Forum member Milanuk explains how the key factor isn’t altitude but air pressure. Milanuk writes: “In simple terms, as you increase your altitude, the density of air the bullet must travel through drops, thereby reducing drag on the bullet. The bullet will drop less if the altitude is higher than it was at sea level. For example, I shoot at two ranges in the Pacific Northwest. Both are at 1000? AMSL (Above Mean Sea Level) or less. To get 100-1000 yards with a Berger VLD 155gr at 2960 fps, I will need 29-30 MOA. Raton, NM, is 6600. AMSL will only require 24-25 MOA for me to do the same. This is a significant difference. It is the barometric pressure, not the nominal altitude that really matters. The barometric pressure will show you the pressure drop at higher altitudes. It will also show you how pressure changes when a front approaches, etc. This can cause havoc with your calculated come-ups. Most altimeters are barometers that measure in feet, not inches of mercury. Milanuk says that it is not altitude per se that is important, but the LOCAL Barometric Pressure (sometimes called “station pressure”) which is. Air temperature and barometric tension are the two most important atmospheric conditions that affect bullet flight. Humidity has a negligible effect. Remember that the barometric pressure reported by the radio or internet may be referred to as a sea-level equivalent. Denver, at 6,000 feet AMSL, if the local pressure in Denver is 24?, then the radio will report that the barometric pressure is 30?. You can bring a Kestrel or mentally correct the pressure of the radio station if you shoot at high altitudes. per 1,000 feet. Trajectory for Bullet fired at Sea Level
Trajectory of Bullet fired at 22,000 feet
JBM Online Ballistics is free to use for experimental calculations. Here’s an extreme example. This printout (generated using Point Blank software) shows two bullet trajectory maps at sea level (0?). One at 20,000 feet and one at sea level (0?). We gave the bullet a velocity of 3000 feet per second and a low 0.2 BC for demonstration purposes. Hornady provides a helpful discussion on Exterior Ballistics that includes the effects of temperature and altitude. Sierra Bullets has a detailed Exterior Ballistics Resource page with many sections from the Sierra Manual (5th and 6th Editions), including Section 3.0: Exterior Ballistic Impacts on Bullet Flight
Section 3.1: Effects on Altitude and Atmospheric Conditions
Section 3.2: Wind Effects
Section 3.3: Effects on Shooting Uphill or Downhill. Example from Section 3.0: “When bullets fly through the air, there are two types of forces that act on them to determine their path (trajectory). The first is the gravitational force, and the second is aerodynamics. There are many types of aerodynamic forces that act on a bullet. These include drag, lift and side forces, Magnus force spin damping force, pitch damping for force, and Magnus cross force. Drag is the most important of all these aerodynamic forces. When the bullet is spin-stabilized, all the other aerodynamic forces are negligible.”
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