{"id":1741,"date":"2023-12-20T18:07:07","date_gmt":"2023-12-20T18:07:07","guid":{"rendered":"https:\/\/gunowner-news.com\/?p=1741"},"modified":"2023-12-20T18:07:07","modified_gmt":"2023-12-20T18:07:07","slug":"what-is-ballistic-coefficient-the-armory-life","status":"publish","type":"post","link":"https:\/\/gunowner-news.com\/?p=1741","title":{"rendered":"What Is Ballistic Coefficient? – The Armory Life"},"content":{"rendered":"


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By Wayne van Zwoll<\/a><\/p>\n

W<\/span>hen I first fired a rifle, B.C. was a distant time predating gunpowder. Now, in shooter chat, it\u2019s a number assigned to a bullet. It has nothing to do with the cartridge case or the powder charge \u2014 except that a bullet\u2019s velocity can affect its BC.<\/p>\n

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Big game bullets developed for controlled upset and deep penetration now feature high-BC profiles.<\/figcaption><\/figure>\n

As slumber is easily induced by reading about BC, I\u2019ll be brief. But a rough grasp of it matters if you wish to hit small targets beyond the range at which you can kill deer offhand with iron sights.<\/p>\n

Basics of Projectile Ballistic Coefficient<\/h2>\n

Stripped bare, BC is a comparative measure of how well a bullet pierces air, or fights the drag set up by its own velocity. For bullets commonly used in small arms, BC is a three-digit number following a decimal. The higher that number, the better a bullet fares battling air resistance.<\/p>\n

\"hornady\n<\/picture>
Hornady\u2019s .30-bore alloy-tipped match bullets have G1 BCs as high as .823 and .878. Fast twist a must!<\/figcaption><\/figure>\n

Before dismissing air as a nothingburger, stick your arm out the window of an automobile rolling down the highway at 60 miles per hour. You\u2019re traveling at just 88 feet per second. But your muscles are taxed holding your arm straight. The car\u2019s movement causes still air to feel like strong wind. But its pace is glacial compared to a bullet\u2019s. A 180-grain softpoint leaving the muzzle of a .30-06 at a standard 2,700 fps is moving more than 30 times faster than your sedan! Were you riding that bullet, your hair would be flat indeed, your eyes watering.\u00a0<\/p>\n

Every bullet decelerates as soon as it escapes the thrust of powder gas \u2014 that is, as soon as it exits the barrel. Air resistance and friction, plus turbulence set up in flight, act as brakes. All the while, gravity pulls the bullet toward earth at the accelerating rate of 32 fps per second. Energy measured in foot-pounds is a function of bullet weight and the square of bullet velocity, so as the bullet slows, it loses its authority.\u00a0<\/p>\n

\"nosler\n<\/picture>
Sectional density contributes to BC, but highest SDs nix pointed noses. For tough game, it is a good trade.<\/figcaption><\/figure>\n

Measuring the speed of bullets, and tracing their path, challenged early shooters. Then in 1537, a bright Italian gentleman named Trataglia suggested in a book that bullets traveled in arcs, a radical idea when many people assumed they flew straight until spent, then fell abruptly to earth. Trataglia\u2019s experiments showed the launch angle that gave bullets their greatest range was near 45 degrees. This is much steeper than the angle that gives a .30-06 bullet its longest reach. Still, Trataglia\u2019s conclusion was valid for his day. At the low speeds of early balls from black-powder muzzle-loaders, gravity had a greater effect than drag on the projectiles\u2019 flight. Modern high-speed bullets, in contrast, are influenced more by drag.<\/p>\n

A century later, studying trajectory for the Venice arsenal, Galileo dropped cannon balls from the Leaning Tower of Pisa. Accepting gravity\u2019s tug as constant, he determined bullet paths must be parabolic, and affirmed 45 degrees as the launch angle for maximum range. But as Trataglia, he ignored drag in his calculations. Compared to the acceleration of gravity, drag on a cannonball dropped from a window was essentially zero!\u00a0<\/p>\n

\"high\n<\/picture>
High-BC bullets face constraints imposed by magazine and throat lengths, case capacity and rifling twist.<\/figcaption><\/figure>\n

Another century on, Englishman Benjamin Robins devised a ballistic pendulum. Firing a bullet of known mass into a heavy wooden bob, also weighed, he calculated the bullet\u2019s velocity by measuring the pendulum\u2019s swing. In the 1740s, few of his colleagues could imagine musket balls streaking away at 1,400 fps! Equally incredible: low readings from longer range suggesting that drag on the balls was 85 times as strong as the pull of gravity! Decades later, drag measurement would bear out Robins\u2019 results.<\/p>\n

Isaac Newton conducted pendulum experiments, too. One of his findings was the universal law of gravitation, which declares that the force of gravity varies with altitude. Sir Isaac\u2019s fundamental laws of mechanics, and his work on calculus (concurrently with Leibnitz), were crucial to the study of ballistics. Newton showed that drag increases with air density and with the cross-sectional area of the bullet. He also demonstrated a relationship between drag and the square of a projectile\u2019s speed. With no way to measure the speed of musket balls, he couldn\u2019t know that drag spikes as they near the speed of sound (1,120 fps).\u00a0<\/p>\n

\"black\n<\/picture>
The author found that match bullets heavy for the bore, with long, pointed ogives, excel at long range.<\/figcaption><\/figure>\n

Invention of the electro-ballistic chronograph in 1864 would absolve mathematicians from using calculus to predict bullet trajectories and would open new investigations into bullet flight.<\/p>\n

Use of a ballistic coefficient became practical late in the 19th century, with adoption of \u201cstandard\u201d bullets. Fashioned after artillery rounds of that era, they had specific shape, weight and dimensions. Early to the party was Germany\u2019s Krupp bullet. Krupp data prompted a Russian colonel, Mayevski, to develop a mathematical model showing its drag deceleration. In 1881, the Gavre Commission in France came up with a similar bullet 1 pound in weight and an inch in diameter. It was 3.28 inches long, with a 1.96-inch shank and a 1.32-inch nose with 2-caliber ogive. Tables by U.S. Army Colonel James Ingalls, published in 1893 and revised in 1917, used this Gavre Type 1 bullet, now known as G1. <\/p>\n

Calculate Ballistic Coefficient<\/h2>\n

Any bullet\u2019s BC is essentially a mathematical comparison of its form and sectional density with those of the standard bullet. Computing BC \u2014 formally \u201cC\u201d with a \u201cballistic\u201d subscript \u2014 is a tedious task.<\/p>\n

\"hunting\n<\/picture>
Most game is killed within the reach of standard softpoint loads. Only at very long range does a projectile\u2019s BC truly matter.<\/figcaption><\/figure>\n

Of various formulas, the simplest is: C=SD\/i, where \u201cSD\u201d is the bullet\u2019s sectional density, or the ratio of its weight in pounds to the square of its diameter in inches, and \u201ci\u201d is the form factor.<\/p>\n

Getting to \u201ci\u201d is no fun mathematically unless you\u2019re a numbers nerd. Think \u201cstreamlined.\u201d The higher its form factor, the easier a bullet cleaves air. Bullets with a long ogive (the nose, from shank to tip) and tapered heel brook less resistance and decelerate less readily than bullets the shape of a soup can.<\/p>\n

Moving (Bullets) Forward<\/h2>\n

In my youth, pointed or \u201cspitzer\u201d bullets with G1BCs of around .400 were considered racy. Many hunters carried .30-30 lever rifles with 170-grain bullets whose flat noses trimmed BCs to half that. The .360 BCs of the pointed 180s in my .303 SMLE seemed high to me. They\u2019re modest by current measure.<\/p>\n

\"28\n<\/picture>
A long-rifle action and magazine accommodate these racy bullets for the 28 Nosler. G1 BC: over .650.<\/figcaption><\/figure>\n

While G1 values are still commonly applied to hunting and target bullets, the G1 standard bullet was faulted shortly after its debut for its rapid deceleration, which changed its ballistic properties. Sleeker shapes evolved. Here with a summary:<\/p>\n