November 15th, 2016

Altitude, Air Pressure and Ballistics — What You Need to Know

Trajectory of Bullet fired at Sea Level

Trajectory of Bullet fired at 20,000 feet

You can do your own experimental calculations using JBM Online Ballistics (free to use). Here is an extreme example, with two printouts (generated with Point Blank software), 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.

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 member Milanuk explains 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.

Note that 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.

It’s important to remember 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 Kestrel, or remember to mentally correct the radio station’s pressure, by 1″ per 1,000 feet.”

If you want to learn more about all aspects of External Ballistics, ExteriorBallistics.com provides a variety of useful resources. In particular, on that site, Section 3.1 of the Sierra Manual is reprinted, covering Effects of Altitude and Atmospheric Pressure on bullet flight.

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September 22nd, 2016

Aiming Techniques for F-Class Competition

F-Class Aiming Long Range Score Shooting
The movie “The Patriot” gave us the phrase “Aim small, miss small”. While that’s a good mantra, aiming strategies for long-range competition are a bit more complicated, as this article explains…

The U.S. Mid-Range and Long Range Nationals kick off tomorrow, September 23rd, in Lodi, Wisconsin. Here are some tips that can help F-TR and F-Open shooters aim more precisely, and achieve higher scores. F-Class ace Monte Milanuk reviews reticle choices and strategies for holding off.

In our Shooters Forum, one newcomer wanted some advice on selecting a reticle for F-Class optics. He wondered about the advantage of Front (first) Focal Plane (FFP) vs. Second Focal Plane scopes and also wondered if one type of reticle was better for “holding off” than others.

In responding to this question, Forum regular Monte Milanuk provided an excellent summary of aiming methods used in F-Class. For anyone shooting score targets, Monte’s post is worth reading:

Aiming Methods for F-Class (and Long-Range) Shootingby Monte Milanuk

600-yard F-Class TargetF-Class is a known-distance event, with targets of known dimensions that have markings (rings) of known sizes. Any ‘holding off’ can be done using the target face itself. Most ‘benefits’ of Front (first) focal plain (FFP) optics are null and void here — they work great on two-way ranges where ‘minute of man’ is the defining criteria — but how many FFP scopes do you know of in the 30-40X magnification range? Very, very few, because what people who buy high-magnification scopes want is something that allows them to hold finer on the target, and see more detail of the target, not something where the reticle covers the same amount of real estate and appears ‘coarser’ in view against the target, while getting almost too fine to see at lower powers.

Whether a person clicks or holds off is largely personal preference. Some people might decline to adjust their scope as long as they can hold off somewhere on the target. Some of that may stem from the unfortunate effect of scopes being mechanical objects which sometimes don’t work entirely as advertised (i.e. one or two clicks being more or less than anticipated). Me personally, if I get outside 1-1.5 MOA from center, I usually correct accordingly. I also shoot on a range where wind corrections are often in revolutions, not clicks or minutes, between shots.

Some shooters do a modified form of ‘chase the spotter’ — i.e. Take a swag at the wind, dial it on, aim center and shoot. Spotter comes up mid-ring 10 at 4 o’clock… so for the next shot aim mid-ring 10 at 10 o’clock and shoot. This should come up a center X (in theory). Adjust process as necessary to take into account for varying wind speeds and direction.

John Sigler F-Class

600-yard F-Class TargetOthers use a plot sheet that is a scaled representation of the target face, complete with a grid overlaid on it that matches the increments of their optics — usually in MOA. Take your Swag at the wind, dial it on, hold center and shoot. Shot comes up a 10 o’clock ‘8’… plot the shot on the sheet, look at the grid and take your corrections from that and dial the scope accordingly. This process should put you in the center (or pretty close), assuming that you didn’t completely ignore the wind in the mean time. Once in the center, hold off and shoot and plot, and if you see a ‘group’ forming (say low right in the 10 ring) either continue to hold high and left or apply the needed corrections to bring your group into the x-ring.

Just holding is generally faster, and allows the shooter to shoot fast and (hopefully) stay ahead of the wind. Plotting is more methodical and may save your bacon if the wind completely changes on you… plotting provides a good reference for dialing back the other way while staying in the middle of the target. — YMMV, Monte

Permalink Optics, Shooting Skills 1 Comment »
October 19th, 2014

How Altitude and Air Pressure Influence Ballistics

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 member Milanuk explains 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.

Note that 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.

It’s important to remember 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 JBM Online Ballistics (free to use). Here is an extreme example, with two printouts (generated with Point Blank software), 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

if you want to learn more about all aspects of External Ballistics, ExteriorBallistics.com provides a variety of useful resources. In particular, on that site, Section 3.1 of the Sierra Manual is reprinted, covering Effects of Altitude and Atmospheric Pressure on bullet flight.

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