In previous blogs we have investigated 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
• Mach is the ratio of a bodies velocity to the speed of sound in a fluid. Mach 1 is about 1125 feet per second in air, in water this is 4,868.77 fps over 4.3 times faster. Mach 2 is 2250 feet per second in air. The importance of Mach numbers is that speeds over Mach 1 a bow wave (bow shockwave) is created, and requires energy to generate. The energy saps the kinetic energy and retards bullet velocity through air. This shockwave is a mini-sonic boom and is the crack heard as the bullet passes your position, it is none-directional down range from the shooter. You know your being shot at, but not the direction. This wave contributes more to the slowing of the projectile than friction with the atmosphere above Mach 1.
• Mass is the amount of matter in a physical object.
• Motion: ballistics is the study of bodies in motion. Sir Isaac Newton was a leader in motion studies and helped to formulize calculus to deal with the mathematics of motion and kinetic energy. Most of the formulas we us in ballistic have been reduced to their algebraic forms, but calculus is used in bullet drop and other calculations related to ballistics.
• Matter is a physical substance. Six states of matter are: solid, liquid, gas, plasma, Bose-Einstein and Fermionic condensates. We are mostly concerned with solids and gases. Propellants convert from solids to gas, and the atmosphere is a gas as well, but all other components we deal with are solids.
• Meplat is French for flat and refers to the point or tip of the bullet. Manufacturing processes either form a flattened point or a rounded point, both are referred as the meplat.
• Ogive is the curve portion of the bullet. Secant ogives is the primary form in bullet manufacture and tends to blend into the body of the bullet. Tangent ogives are sharper, coned shape ogives and don’t transition in the bullets body as smoothly as the secant curves do, but are popular in low-drag bullet designs.
• Sectional Density (SD) is the component that takes the mass and caliber or area into consideration. The formula is:
SD = Wt/7000 x d2
Where: Wt is the bullet weight in grains, the 7000 converts the grains to pounds and d is the diameter squared. The SD is independent of the bullets construction and shape. All .30 caliber bullets of 150 grains will all have the same sectional density, regardless of form, shape or materials.
• Speed of sound is simply the speed at which a sound wave propagates through an elastic medium per time. In dry air at 0 oC or 32 oF 742.5 mph or 1089 feet per second. At 20 oC or 68 oF is 767 mph or 1124.93 feet per second. This is sonic, supersonic above and subsonic below. As noted in air the speed of sound is temperature dependent.
• Spitzer is German for pointed.
• Standard Projectile. Ballistic coefficients are calculated compared to the standard projectile. The projectile is 3.28 calibers long, with a 2-caliber radius ogive. A bearing surface, or body length of 1.96 calibers and a point length of 1.32 calibers, with a flat base and a BC of 1.00 and this is the G1 function standard. G7 is a very-low-drag bullet with a 10-caliber secant or tangent ogive, and a 7.5o boat-tail. The G7 is 4.280 calibers is length.
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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.