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Ballistic Coefficient Terms & Concepts – From Standard Temperature Pressure to Weight

In previous blogs Shooting Range Industries have explored the advantage of the Ballistic Coefficient (BC) in BC-Part 1, and defined some key terms and concepts as we can see that velocity plays a key role in long range precision engagements. Before we explain how BC is determined, we have some more key terms and concepts to discuss.

Ballistic Coefficient Terms & Concepts

STP = Standard Temperature Pressure as ballistics are dependent on atmospheric conditions of temperature and air pressure a standard is set for test conditions, Pressure = 1 Atm (atmosphere, or about 14.70 lbs/in2 (which is sea-level atmospheric pressure), while scientific studies dictate 0 oC (273.15 K or Kelvin), but ballisticians use 15 oC (Celsius) or 59 oF (Fahrenheit). These standard are closer to real-world shooting conditions. The behavior of gases is well understood and local conditions that vary from the standard can be calculated back to the STP. Terminology it should be noted that most of our ballistic terms come from the French, as they and Germans were the pioneers in ballistic studies. Occasionally an Italian term will crop-up. Free flight projectiles, bullets and unguided rockets, fall under the term ballistics. Powered and guide bodies like planes and missiles use things like drag function and drag coefficients where ballistics utilizes the ballistic coefficient.
Transonic Ballistics. When the bullet transitions from supersonic to subsonic the pressure center moves towards the tip of the bullet, affecting the stabilization of the projectile. The transonic region is between Mach 1.2 down to Mach .8. Some bullets have been known to destabilize through the transonic region and tumble destroying accuracy. Transitional ballistics is only of concern to the long-range precision shooter. Most common long-range calibers maintain supersonic through at least 1000 yards and beyond.
Units: most of us who are into shooting sports are old enough to have been introduced to the English/American imperial units of measurement in school. As the rest of the world marches to the tune of the metric system most shooters are more familiar with US units than SI (Society International in English, is the international standards organization) units, which are based on the metric system. Science and engineering use SI units, we’re drifting that way, but US ballistic discussions generally use feet, inches, Fahrenheit, and yards. Weight, not mass and predominately the grain, 7000 grains to the pound. Officially the Bureau of Standards defines all traditional US/English units in SI units.
Weight is mass in a gravitational field with interaction between two or more bodies of matter. Acceleration due to gravity in US units is 32.17 feet per second per second for the Earth. Weight is a force and is defined as mass times acceleration, the aforementioned 32.7 fps2. The slug is the imperial unit of mass; 1 slug times gravity acceleration = 32.17 pounds. Of course, the metric systems uses mass, as it stays the same regardless if you’re in the earth’s gravitational field or the moons, as weight is less on the moon.

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Stay tuned for the next blog in our series of ballistic coefficients. Contact Shooting Range Industries to learn more about hiring us to design and build your own a custom shooting range for firearms practice and training.

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