# Sectional density, ballistic coefficient and wind drift

Discussion in 'Long Range Hunting & Shooting' started by dicktaylor, May 26, 2007.

1. ### dicktaylorMember

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May 21, 2007
Is ballistic coefficient the best measure of a bullet's ability to buck wind? What about velocity? How does it fit in?

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Ballistic Coefficent is determined by the amount of velocity lose that a projectile has as it travels down range.

Velocity drop also determines the effect of gravity on the bullet down range and also the effect the wind will have on a bullet down range.

To greatly simplify things, the faster a bullet velocity drops, the more the bulllet will drop due to gravity and the more the wind will push the bullet off course.

BC is just a number we can use to get an idea of how a bullet will perform ballistically and also helps us compare it to other bullet designs.

Kirby Allen(50)

3. ### Aragorn50Member

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Ballistic coefficient and velocity are both important. Ballistic coefficient is a mathematical factor that allows us to use formulas to calculate bullet path. It can best be thought of as a measure of how much "drag" a projectile has as it travels though the atmosphere. Factors that affect ballistic coefficient include:
1. Sectional density: (how much mass or weight a projectile has compared to it's diameter) Thus all other things being equal, a longer projectile of the same material will have more weight and a higher sectional density.
2. Shape or form factor: This includes both the front and rear of the projectile. Projectiles with a longer ogive will have less drag. Secant ogives are somewhat more efficient than tangent ogives. The length and angle of the boattail and whether or not it is rebated also play a role.

The essential concept has to do with time of flight(TOF). The shorter the TOF, the less time gravity and wind will have to alter the bullet path. Thus initial velocity and rate of velocity loss are both important. Bullets with higher BC's will lose less velocity than bullets with a lower BC. This makes the TOF shorter and thus gravity has less time to cause the bullet to drop, and wind has less time to push it off of it's initial path. Higher BC = less drag = shorter TOF all other factors being equal.

4. ### MikecrWell-Known Member

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There is also BC-vs-Velocity to consider depending on your cartridge and use. This, putting aside accuracy, recoil, barrel life, energy, etc.

If you could choose for instance 243AI 55gr@3850fps -vs- 105gr@3000fps, and you only intend to kill varmints out to ~300yds, the faster 55gr might be flatter and straighter for you.
Just opposite for 600yds..

Play around alittle with ballistic software &amp; see.

5. ### Jeff In TXWell-Known Member

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Ballistic coefficients are the measurement of a bullets ability to overcome air resistance in flight. There is also an inverse proportional factor to deceleration, the higher the BC the lower the deceleration factor. In a nut shell the more aerodynamic the bullet the less air resistance it will encounter in flight.

The original G1 drag model and calculations are based on a 100+ year-old Russian Colonel’s calculations using a 1 inch 1 pound round nose projectile as its basis. Bullet manufactures still use the G1 drag model today because they yield high BC numbers. The theory is the higher the BC the more bullets they’ll sell.

Coefficient drags (CD's) are much more accurate for predicting the flight of a bullet over long ranges.