How to determine ballistic coefficient?


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gunnutery
February 27, 2010, 01:56 PM
I did a quick search and didn't find anything about this subject.

How does one determine what their ballistic coefficient is? Is it based upon the (A)rifle's barrel length, or (B)caliber or (C)how many times I can hop on one leg??? Or... is it both A and B?

Everytime I go to a ballistic calculator, it asks for a ballistic coefficient. Is there a SIMPLE calculation for finding it?

Thanks for any input. NOTE: I'm not a math wiz, so please be gentle.

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Vicious-Peanut
February 27, 2010, 02:02 PM
I go the the bullet manufacturers website. :D

Its based on the length of the bullet and caliber.

EddieNFL
February 27, 2010, 04:58 PM
Try this:

http://www.uslink.com/~tom1/calcbc/calcbc.htm

gunnutery
February 27, 2010, 05:58 PM
Both helpful suggestions. Thankyou. Eddie, I'm not sure how soon I'll be able to attempt to try such measurements, but I'll give it a go.

There are really only a couple rounds that I need a BC on, both are from Federal, and Federal doesn't seem to want to list their BC's. They did have ballistic charts though and that will be helpful for now.

Thanks again.

rockit
February 27, 2010, 06:33 PM
Federal has a really good ballistics program with BC's on their website for their ammo. Just download it to your computer.
http://www.federalpremium.com/resources/ballistics_application.aspx

Carne Frio
February 27, 2010, 06:44 PM
An explanation:

http://en.wikipedia.org/wiki/Ballistic_coefficient

06
February 27, 2010, 06:54 PM
My son is going "ga ga" over BCs and wants a 280. Supposedly it has a 6.?? BC over most others having only 5s of various tenths. Sure will be glad when he gets it so I can shoot right with him using my trusty old '06. LOL, sure hope I can even come close as he is a crack shot. He and his sniper buds play at Ft. Bragg right often. I tell him he needs the practice if he intends to beat his "old Paw".

Zoogster
February 27, 2010, 07:06 PM
Ballistic coefficient is dependent on the projectile.

It is a measure of a projectile's ability to overcome air resistance.

There is a couple things at work in it.

First the diameter of the projectile or frontal area is going to determine how much area is directly resisting the air.
If it tapers towards the front the sides of the projectile will come into play as well.

The shape is going to also be key. A flat nose is going to have much greater resistance than a round nose. A JHP more resistance than a FMJ.
This is the reason a spitzer shape is typically used for modern rifle calibers. It has very little resistance.

Another prime factor is the mass. The exact same shaped bullet scaled up will have greater mass, and so a better ballistic coefficient.
So for example even though the .50 BMG is a scaled up .30-06, if both projectiles are otherwise identical in proportion the .50 will still have a better BC.


Assuming identical mass a short, wide, and flat (or JHP) round would have a horrible BC, and a long, thin, and pointed (or spitzer) would have a good BC.



The round with the best BC can however also not be the most optimized for stopping power.
If it has little resistance in air it will clearly have less in tissue as well. Spitzers slightly compensate for this because they become unstable and flip at least once after losing stability.
Expanding rounds compensate by increasing diameter and resistance after hitting tissue.
Simply one generally wants great tissue resistance (within penetration considerations), but limited air resistance.




Measuring the BC of a round can be a pain. You need to precisely measure the angles of the bullet, including the pocket of a hollow point. A rough estimate is a little easier. Then precisely measure it.
A related area is knowing whether strange eddies are formed around the bullet during flight. These can disturb flight even more than the simple equation will show.

If the manufacturer lists the BC is makes things easier.

Shung
February 27, 2010, 08:47 PM
Measuring the BC of a round can be a pain. You need to precisely measure the angles of the bullet, including the pocket of a hollow point. A rough estimate is a little easier. Then precisely measure it.

can't you measure it by knowing it's weight, diameter, and the speed at 2 indicated distances (ex: 0 and 100yards) ? Of course, you'd have to know the temperature, pressure, altitude etc to precisely calculate it..

I have a freeware (PointBlank.exe) that claims to be able to calculate it with these datas inputed.

Kernel
February 27, 2010, 09:47 PM
BC's change depending on which ballistic model you use.

One way to estimate at BC is calculate the sectional density and multiply it by a form factor: 1 for a typical boattail spritzer, 0.80 for a flat nose, 1.1+ for something aerodynamic. Will usually get you to within plus/minus 10%.

Sunray
February 27, 2010, 09:55 PM
"...doesn't seem to want to list their BC's..." The BC isn't terribly critical.

Kernel
February 28, 2010, 01:17 AM
The BC isn't terribly critical
True. Not until you're getting out to 500 yds and beyond.

Zoogster
February 28, 2010, 05:04 PM
can't you measure it by knowing it's weight, diameter, and the speed at 2 indicated distances (ex: 0 and 100yards) ? Of course, you'd have to know the temperature, pressure, altitude etc to precisely calculate it..

I have a freeware (PointBlank.exe) that claims to be able to calculate it with these datas inputed.

Pretty close.
The weight and diameter alone are not enough for precision because they don't tell you the shape. Even the same shape can be quite different with different angles. The angle on the surface of the bullet is going to change the airflow. That changes the BC. A slight angle change will dramatically change the outcome. Just like slightly changing the nose on a jet has a major impact on the drag.

If you know the actual speed at two separate distances though that tells you how much speed it lost to air resistance in between. So combined with the weight and diameter it should actually be quite good (depending how well the software calculates different drag at various speeds and how precise the equation is.)
You would know about what the BC is, but not why.

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