One of our readers asked “What effect does altitude have on the flight of a bullet?” The simplistic answer is that, at higher altitudes, the air is thinner (lower density), so there is less drag on the bullet. This means that the amount of bullet drop is less at any given flight distance from the muzzle. Since the force of gravity is essentially constant on the earth’s surface (for practical purposes), the bullet’s downward acceleration doesn’t change, but a bullet launched at a higher altitude is able to fly slightly farther (in the thinner air) for every increment of downward movement. Effectively, the bullet behaves as if it has a higher ballistic coefficient.
Forum members Milanuk and Catshooter explained that the key factor is not altitude per se, but rather air pressure. Milanuk writes:
“In basic terms, as your altitude increases, the density of the air the bullet must travel thru decreases, thereby reducing the drag on the bullet. Generally, the higher the altitude, the less the bullet will drop. For example, I shoot at a couple ranges here in the Pacific Northwest. Both are at 1000′ ASL or less. I’ll need about 29-30 MOA to get from 100 yard to 1000 yards with a Berger 155gr VLD @ 2960fps. By contrast, in Raton, NM, located at 6600′ ASL, I’ll only need about 24-25 MOA to do the same. That’s a significant difference.
One thing to remember — it is the barometric pressure that really matters, not simply the nominal altitude. The barometric pressure will indicate the reduced pressure from a higher altitude, but it will also show you the pressure changes as a front moves in, etc. which can play havoc w/ your calculated come-ups. Most altimeters are simply barometers that read in feet instead of inches of mercury.”
Catshooter adds: “As Milanuk states, it is NOT altitude, but the LOCAL barometric pressure that is key. The two atmospheric conditions that effect bullet flight are air temperature, and barometric pressure - the humidity has so little effect that it need not be considered.
One important thing to remember is that the barometric pressure given on the radio station has been corrected for sea level. So in Denver (at 6,000 feet amsl), if the local pressure is 24″, the radio will report the barometric pressure to be 30″. If you do high altitude shooting at long range, bring along a Kestral, or remember to mentally correct the radio station’s pressure, by 1″ per 1,000 feet.”
You can do your own experimental calculations using FREE Point Blank Ballistics software. Here are two printouts, one showing bullet trajectory at sea level (0′ altitude) and one at 20,000 feet. For demonstration sake, we assigned a low 0.2 BC to the bullet, with a velocity of 3000 fps.
Trajectory of Bullet fired at Sea Level
Trajectory of Bullet fired at 20,000 feet
Here’s a useful resource on External Ballistics if you want to learn more about the effects of altitude and barometric pressure on bullet flight.
A Universal Decapping Die belongs in every reloader’s die inventory. This simple, inexpensive tool allows you to remove primers from any size case, without altering the case body or neck. This die can be useful if you have very dirty brass that you plan to tumble or ultrasonically clean before sizing and loading. If you decap BEFORE ultrasonic cleaning, the ultrasound will clean out the primer pocket. By cleaning before you run the brass in your full-length die, you keep the FL die cleaner and reduce the risk of scratching the inside of the die.