Do balistic coefficents change with velocity???

clarkivan

I was having a conversation with someone recently, and the topic of calculating BC came up. I have always been under the assumption that BC changes, all though slightly, with respect to velocity. I am just wondering if any of you guys can explain how or if this happens...

thanks

IC

Death by projectile

oneshy

I've only recently researched this, and here is how I understand it. The BC is determined by the shape, weight, caliber and general composition of the bullet. As an example, the following Speer bullets all have the same BC; .263-140gr SP, 277gr BT, .284-145gr BT, .284-160gr SP. This is used to predict the expected bullet path, drop, time of flight, impact point,energy etc., at given velocities. So the BC is actually the constant factor used for comparison at different velocities. Bullet man. include the BC in their specs so the shooter can then calculate expected performance with a mathematic formula or as a reference to Ballistic Charts.



Edited by - oneshy on 08/20/2002 13:29:15

flatdog

Hello clarkivan,

I believe that the B.C. changes with velocity i.e. the coefficient of drag.
I did a quick GOOGLE SEARCH for ballistic coefficient +change and came up with [ 1-10 OF 4790 in 0.36 sec. ]. That should keep you busy for a while.
flatdog out.

" Love is what goes on between a man and a .45 pistol that won't jam."

Tazmutt

I believe, and agree with oneshy, that BC is a fixed value determined soley by the bullet construction and not related to or affected by velocity.

steve45

My Sierra reloading manual shows different ballistic coefficients for the same bullet at different velocities. Example a .270 bullet 130 gr. spitzer boatail has a bc of .450 at 3100 fps, .440 at 2400, and .372 at 1600 fps. I have found in the text that one method of determining bc is wrong because it does not change bc with different velocites. I have not yet found out why it would change but will post it when I do. My Sierra manual is the second edition.

rameleni1

BC is constant with the same bullet (drag). Velocity only changes the arc, or drop of the bullet.

Rameleni1

Tazmutt

Steve - If that be the case, I stand corrected ... My older Lyman reloading manual does not list them this way. I guess it makes sense though that the higher the velocity, the higher the BC would be.

Shootist3006

As best I know, BC DOES change with velocity. Go to http://www.sierrabullets.com/bullets/bc.cfm and check it out.

Quod principi placuit legis habet vigorem.Semper Fidelis

clarkivan

Hey guys

I spent quite a little time researching this... I couldn't find too much on the net that was in depth enough for me but did find some info in my Dynamics of physics textbook. The math behind this is quite perplexing, I have taken a lot of math and physics classes and it is beyond me... Research has been done on ballistics since about 1888 and research is still being done. The explanation is far to in depth for me to explain fully, so I will give you the gist of it... No equation that has been derived can be used to determine BC exactly for a given velocity! There, I said it, read on if you want...

BC is calculated using a very simple equation that has extraordinarily complicated variables derived from undeterminable mathematics equations. Therefore tables are used to give generalized variables for the BC equation.

The following are some of the complications involved in calculating BC.

I have deducted from what I have read that the BC is constant if and only if the velocity is constant, and if it flies in a straight line. Projectiles DO NOT fly in a straight line; rather they rotate about an undeterminable axis at an undeterminable parabolic frequency ("wobble"). The wobble is created due to a small lift force acting on the nose of the projectile. The faster the bullet flies the faster the rotation, thus dissipating the "lift force" about a smaller axis. If the wobble is too great the bullet will tumble through the air. This is why bullets are rotated by a rifled barrel, at varied twist depending on velocity and size of the bullet...

With this being the case the parabolic frequency ("wobble") of a higher velocity bullet is much smaller than for a slower bullet. The faster rotation causes the "lift force" on the projectile to be transferred into a smaller rotational frequency. Therefore a slower bullet will rotate slower and the rotational frequency is larger causing a higher reaction of the "Lift force". So, the slower bullet will "wobble" more making the BC lower.

Therefore as a projectile slows down the BC decreases...

It is kind of like sticking your hand out the window of a car and rotating your hand in the wind, put your hand up and the wind will catch it but if you put it into the wind not as much catches it. (This is very hard to explain!)

The second reason that the BC changes is due to the speed of sound, BC changes dramatically around the speed of sound. From what I deducted a harmonic wave is created around the bullet, like an envelope. Once the bullet breaks the sound barrier the bullet escapes the envelope and the BC increases dramatically. (Around 1000 fps)

Another factor that changes the BC is the tip design or disfiguration, and or disfigurations form the rifling. This can have adverse affects on the "wobble" causing the BC to decrease drastically.

The only true way to determine BC is on a radar range, and track a projectiles path for the entire given velocity range. Bullet manufactures do not have the time and money to spend on research like this. So they publish BC that will be "close" to what they should be, but they could be off by quite a little depending on each rifle and the rate of twist generated at different velocities.

For the rest of us, a chronograph will work close enough for what we need it for.

In closing, the BC for a given bullet is only an approximation. Each and every bullet has a different BC at every possible velocity. The only way to determine BC for the bullets that you are shooting is to chronograph bullets at different distances for multiple shots...

Hope this didn't bore to many of you

Ivan

Death by projectile

.280 freak

Go to Google, type in "sectional density ballistic coefficient" (without the "", of course).

LOTS of stuff out there on this subject, but the quick answer is, yes, B.C. changes with speed of a bullet.

Sectional density, being the ratio between bullet diameter and weight, being a constant, does not. For example, any .30 caliber bullet weighing 180 grains will have the exact same sectional density as any other .30 caliber bullet of the same weight, regardless of shape, from round nosed to the sleekest spire point.

As the previous poster said, the math involved with figuring B.C. can be rather intimidating. In the simplest terms, B.C. is the relationship between sectional density and how aerodynamic a particular bullet shape is, and the figure will change as the bullet speed changes.

varmit hunter

B.C., Not only changes with velocity. It is also affected by altitude, and barometric pressure. It is the best thing we have for it's intended purpose.

The most important things, Are not things.