Ballistic coefficient: What is it? Part 2

by Tom Gaylord
Writing as B.B. Pelletier

Part 1

This report covers:

  • Review
  • Today’s discussion
  • Round balls
  • Conical bullets
  • Smokeless powder
  • A big point
  • Shape
  • Round balls — again
  • The bottom line

I’ve taken 11 months to return to this subject of ballistic coefficients (BC). That was in spite of some tremendous interest in Part 1 of this report last May.

I’m purposely avoiding all discussion of mathematics, which is difficult, since ballistics is a discipline that heavily employs mathematics. But I’m not qualified to write about the math; and, more importantly, I know that 99 percent of my readers would be turned off if I were to write the report that way.

Review

Last time we learned that the BC of a pellet:

• Is an extremely small decimal fraction compared to the BC of a conical bullet.
• Varies with the velocity of the pellet.
• Varies with the shape (form) of the pellet.

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Ballistic coefficient: What is it? Part 1

by Tom Gaylord, a.k.a. B.B. Pelletier

This report addresses:

• Definition of ballistic coefficient (BC).
• How are BCs determined?
• Bullets and pellets have an additional factor.
• BCs are not constants.
• BC is an expression of how much velocity is lost in flight.
• How to cheat the BC numbers.

If ever there was an elephant in a room full of airgunners — this is it! Ballistic coefficient. It seems like everybody talks about it, but what does it mean?

Definition
Ballistic coefficient (BC) is the measure of a ballistic projectile’s ability to overcome air resistance in flight. It’s stated as a decimal fraction smaller than one. When diabolo pellets are discussed, the BCs are very low numbers in the 0.010 to 0.045 range because diabolos are purposely designed to slow down in the air. Their wasp waists, flared skirts and hollow tails all contribute to very high drag that rapidly slows them down — much like a badminton birdie. Lead bullets, in contrast, have BCs between 0.150 and 0.450.

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