Ballistic Coefficient / Sectional Density . . .

I see that Sierra lists BC depending upon velocity. What's BC all about? Why do I need to consider it?

Sectional density is ratio of length to width? Or am I way off on that one?

Thanks!

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PowderBurns Black Powder / Muzzle Loading Forum:

www.hotboards.com/plus/plus.mirage?who=powderburns

Very simple:

At the same velocity, bullet with better(higher) BC will fly flater than bullet with lower BC. It means less bullet drop, less velocity loss, higher energy at same distance, less wind drift etc. . .
BC is not constant, at different velocities, BC is changing. It is not accurate to say that higher velocity=lower BC! Yes, at very high vel. is BC lower, but at certain range BC of some bullets is vacillating.

SD is relative to weight to cross-section ratio.

(weight in grains)
SD=----------------------
7000x(bullet dia.)^2

7000 is there because transfer grains to pounds

Jiri

[This message has been edited by Jiri (edited 04-21-2001).]

I found this online:

http://www.lostriverballistic.com/LRB_Questions.htm

What is Ballistic Coefficient (BC) and what is sectional density (SD)?

A ballistic coefficient (BC) is the ratio of the sectional density of a bullet to it's coefficient of form (I). A BC is in fact a numerical representation of the ability of a bullet to fly.

Greater the ballistic coefficient, the flatter the trajectory and retention of velocity. Ballistic coefficient may vary depending upon velocity.

Sectional density (SD) is a bullet's weight, in pounds, divided by the square of it's diameter, in inches. SD is a numerical representation of a bullet's ability to carry it's momentum forward. The higher the SD, the higher the weight per cross-sectional area. Shape makes no difference in the sectional density. For example, all 30 caliber (.308) bullets that weigh 180 grains have the same sectional density, .270, regardless of their shape.

This link provides a java script format for calculating BC by three different methods.

(You can archive this info in the AccurateReloading FAQ's.)

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PowderBurns Black Powder / Muzzle Loading Forum:

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[This message has been edited by PowderBurns (edited 04-21-2001).]

Hope this looks O.K. Comparing two .30 calibre bullets, a 180 grain pointed bullet with a B.C. of .4 and a 180 grain round nose with a B.C. of .266.

Muzzle Velocity 2700
Ballistic Coefficient .4
Sight Height (In.) 1.5 Start at (Yd.) 100
Sight-in Range 200 Maximum Range 400
Range Velocity TOF Drop Trajectory Wind @
0 2700 0.000 0.0 -1.5 10 MPH
100 2479 0.115 2.5 2.1 0.9
200 2267 0.242 10.7 0.0 3.6
300 2067 0.381 25.6 -8.8 8.4
400 1878 0.533 48.6 -25.8 15.6
Hit Enter for the Menu

Enter Velocity or H for Help or P to Print last run or M for Menu.
Muzzle Velocity 2700
Ballistic Coefficient .266
Sight Height (In.) 1.5 Start at (Yd.) 100
Sight-in Range 200 Maximum Range 400
Range Velocity TOF Drop Trajectory Wind @
0 2700 0.000 0.0 -1.5 10 MPH
100 2371 0.118 2.6 2.4 1.3
200 2066 0.254 11.4 0.0 5.6
300 1786 0.410 28.3 -10.5 13.6
400 1534 0.591 56.2 -31.9 25.9
Hit Enter for the Menu

Bye
Jack