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Re: What is ballistic coefficient ?

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21 October 2004, 13:58
WSMguy
Re: What is ballistic coefficient ?
And all this time, I thought the higher the BC number, the better the bullet flew through the air!!
21 October 2004, 12:03
DigitalDan
I might embellish on that a bit. ALL BC's change in flight as a function of velocity, OR, if you want to take the modern position, Drag Coefficents change as a function of Mach number. Generally speaking Boat tail bullets have a lower Cd at higher mach and vice versa. I'm not certain that is the case with flat base bullets but think it probably is.

BC is a very slippery concept in my eye because there are so many variables at play. A bullet fired in a 1:10 twist will likely have a higher BC than the same bullet fired in a 1:8 twist because of yaw of repose issues. Variations in powder residue will cause changes as well. A minor aside regarding the Hi/Lo BC vs. Velocity mentioned by Eldeguello: I'm thinking that it's not so much form that promotes high or low BC at transonic/subsonic velocities, but how the form reacts to the transition due to base design and gyroscopic stability factor(s). The highest drag regime a bullet has to deal with exists between Mach .7-1.3. Base design deals with a very small portion of total drag, and the effects would necessarily be more pronounced in this regime. Having said that, I note that bullets typical of the 45-70 in the heavier weight ranges fare quite well in their digression through that speed range. I note also that many boat tail bullets have very good drag characteristics at subsonic velocities, something the .300 Whisper takes full advantage of. Point is that the difference in total drag between the two at that point is not significant. Perhaps the biggest issue regarding the boat tails is Gs, and what is happening to them at long range and transonic velocities. This last paragraph is my opinon and is to a large extent unsupported by any specific reference I can recall.

Babble Off.
21 October 2004, 16:17
DigitalDan
Well that's true if they flap their little wings really hard for you.

I have however thrown a number of them in the trash can, and after reviewing my records of this, can find not discernable difference in drop, regardless of BC.
21 October 2004, 21:50
rpsmamasse
Are you sure on the 29.58 in. Hg? I thought standard atmosphere was Sea Level, 59 deg. F and 29.92 in. Hg

Or is the standard atmosphere for aviation different than for shooting sports?

Later,

Ross
22 October 2004, 01:06
Chris F
Quote:

You should know that the B.C. of any bullet varies depending upon the velocity it is travelling, and interestingly, some have higher B.C.s at higher velocities, and some have higher B.C.s at LOWER VELOCITIES! So as a bullet leaves the muzzle and slows down, its' B.C. changes in flight! Some get better, some get worse!



BC's only appear to change with velocity because most bullet manufacturers are using an imperfect "model" to peg their bullet's performance to...the G1 Projectile. Sierra was on of the first to use different G1 BC's to predict bullet peformance at different velocity ranges (because they are afraid to go to the appropriate G model!).

All the BC shifting would go away if the manufacturer's would just use the appropriate "G" drag curve.

Here's the best analogy I could come up with; Predicting bullet flight with the G1 Drag Curve is like trying to draw an SUV while looking at a pickup truck. You'll get the front end right, but you'll have to have the eraser handy as you're drawing the back end as you'll be making corrections off an imperfect model. It would be best to have an SUV to look at while drawing an SUV. Similarly, it is best to pattern bullet drag against that of a similarly shaped bullet's Drag Curve.

It is however correct to say that Drag Coefficients (Cd) change depending on velocity since Cd is a discreet measurement all it's own and not modeled on a surrogate curve.

Oh yes, those that say BC's are pulled out of thin air by the bullet manufacturers should think back a few years when Sierra had to adjust the BC on the 30 cal 168's, because there was a measurement error on the distance the velocities were being captured at downrange. At least Sierra derives theirs empirically. I can't speak for the others.