Form factor and drag coefficient the same??

[Mr.Knowit all]

Hello,
I have a question that I hope someone could help me with. Is the form factor and drag coefficient referred to as the same?

Sorry if that is a ridiculous question.
Thanks

 

[Ballisticboy]

To obtain the drag coefficients in the drag law of the bullet you are interested in multiply the values in the reference drag law by the form factor. This will give you the drag coefficient values used in the trajectory modelling which uses BC as its basis..
It will not be the exact drag law for the bullet however as the reference drag law will almost certainly be a different shape to the drag law for your bullet. This is why individual drag laws for each bullet design which are the correct shape are more accurate at long ranges.

 

[dcali]

No, they're not the same. You can think of a form factor as the efficiency of the design relative to a standard drag function (G1, G7, etc). The drag coefficient can be thought of as the contribution of the bullet's shape to the BC. You want both numbers to be low.

I don't find the form factor to be a terribly useful number as a shooter.

 

[JPeelen]

The ballistic coefficient is an index figure, combining sectional density and air drag (due to bullet shape). It is an aid invented more than a century ago, when computing was an extremely laborious undertaking. Having available one single, very large table (velocities, time of flight, etc.) computed for BC 1.0, it made it possible to quite simply compute velocities etc. for other bulllets with a different BC. A BC higher than 1 tells you that the bullet is better, because its remaining velocity at a distance is higher. Two very different bullets (say 12.7 mm and 8.6 mm) will have exactly the same [computed] trajetory if both have the same BC (and muzzle velocity). The entire procedure only works for flat trajectories, as was learned the hard way in WW1. And a given table is tied to a single drag law.

Today we have computers and it is very easy to include any desired drag law into trajectory computations. Here the form factor comes into use. For example, 0.8 tells you that the bullet drag is only 80 percent of the drag law in question. Contrary to BC, a lower form factor is better.

In short: the BC is an index figure that merges sectional density (diameter, weight) and bullet shape (air drag).
The form factor is an index figure of the air drag alone.

 

[Mr.Knowit all]

Thank you.

 

[243Mendoza]

Thank you.

Drag coefficient is a function of shape, Mach number, yaw angle and Reynolds number (inertial forces/viscous forces).