November 27th, 2009

Ballistics Factors: Altitude and Air Pressure

Suunto AltimeterOne 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 explain that the key factor is not altitude, but rather air pressure. Milanuk writes:

“In basic terms, as your altitude increases, the density of the air the bullet must travel through 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.”

As Milanuk states, it is NOT altitude per se, but the LOCAL barometric pressure (sometimes called “station pressure”) that is key. The two atmospheric conditions that most effect bullet flight are air temperature, and barometric pressure. Normally, humidity has a negligible effect.

One important thing to remember is that the barometric pressure reported on the radio (or internet) may be stated as a sea level equivalency. 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 the JBM Online Ballistics Program (free to use). 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.

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November 27th, 2009

Black Friday Bargains on Top-Rated Hardware

It’s Black Friday, the busiest shopping day of the year. That means there are some spectacular deals available out there on name brand electronics and optics. You can fight the crowds at the malls, or shop online from the comfort of your home. Here are some great deals we found on Amazon.com.

Canon SX20-IS Digital Camera, $349.00
The Canon SX20-IS is an amazing still and video camera for the price. It takes still images up to 12.1 megapixels, using a 20X optical zoom. As you’d expect from Canon, it offers built-in image stabilization and outstanding color and contrast. But here’s what makes this camera special — it shoots HD movies in 16:9 (wide) format, plus SD movies in standard 4:3 format. And the movies are stored to an SDHC memory card, which you can easily transfer to your computer. Heck of a camera for $349.00 with FREE Shipping on Amazon.com.
Canon PowerShot SX20IS 12.1MP Digital Camera with 20x Wide Angle Optical Image Stabilized Zoom and 2.5-inch Articulating LCD

Canon SX20-IS

Samsung 10″ Netbook computer, $339.99 (Windows XP)
This is the netbook used by your Editor. It has a very sharp screen and a battery rated for 9 hours. I’ve been able to get about 7.5 hours run-time from the battery. I really like this unit because it has a great keyboard (probably the best in class) and a rubberized shell that adds to durability. The built-in networking is excellent (it automatically logs on to nearby Wi-Fi hubs), and this computer isn’t stuffed with useless “bloatware” you have to remove. Note, if you want/need Windows 7, look for another brand, such as the latest Toshiba.
Samsung GO N310-13GB 10.1-Inch Midnight Blue Netbook – 9 Hour Battery Life

Samsung Go Netbook

Manfrotto 410 Geared Head, $196.99
If you have an expensive spotting scope, this geared head is well worth the money. It permits you to make precise elevation or tracking (horizontal) adjustments without getting way off target. With most tripod heads you have to fiddle with a bunch of controls to move the scope, but then it usually goes too far and you have to start all over again. With the Manfrotto 410 head you can move the viewing point smoothly, in very small increments. Want to raise your view up 1 MOA at 1000 yards, without touching the horizontal position? No problem. Once you get one of these geared heads you’ll wonder how you lived without it.
Manfrotto-bogen 410 Compact Geared Head with Quick Release – Supports 11.1 lbs

Manfrotto 410 Bogen 3275 geard head

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