Home Forums Hunting & Shooting Hunter's Campfire Ballistic Coefficient Question

Aw shucks! -blush-

Vaughn’s book is a treasure.

We’re getting our geek on now.

Gordons Reloading Tool.

Download it. Install it. Use it.

Don't you have an MS in physics?

Actually I have a undergraduate degree in physics and I was about 3 months from finishing an MS when the navy ended my shore duty early and sent me back to sea for 2 1/2 years. That was back when there wasn’t all the distance and online opportunities of course. That was quite a while ago though lol. Most of my studies were focused more towards the nuclear topics. Reactor physics etc. I do like thinking about the ballistic physics like this stuff. Thinking about a bullet flying through the air is a little different than starting with a quantum mechanical wave equation and 50 pages of calculus and differential equations later deriving the ideal gas law.

That was a tease, like what a high school cheerleader does.

I must say though, I did get out of it that I was right after all....(as if I ever doubted myself...).

Ballistic Coefficient does involve mass, and is also not just a mathematical measurement of how slippery a bullet is. That variable is the drag coefficient which is a big part, but not all of a ballistic coefficient.

That's right, now I remember what you told me.

The general public mostly doesn't understand there are different kinds of physicists and mathematicians just as there are different kinds of medical doctors.

Exactly mass is definitely a part of BC

Well, it's not just a matter of a single factor. Wind strength is a factor, velocity, shape of projectile, etc. Given the same projectile weight and velocity, projectile calibre and shape remains a factor.

Like the constipated mathematician who worked it out with a pencil?

Bb

Doesn't the Barrett 416 have the highest BC of any shoulder fired weapon in the world?

It has to do with the shape of the bullet and how efficiently it flies thru the air overcoming the friction of the air.

Bullets that are heavy, long and have minimal wind resistance have higher ballistic coefficients. You take the ballistic coefficient and the velocity and plug it into a formula to determine the bullet trajectory.

Spot on! Increased twist definitely increases BC and aids in the supersonic transition to subsonic at very long range

I understand what you're saying and why, but the way you've said it is problematic.

BCs can be derived from the physical characteristics like shape, mass, density, etc., and that's what is typically published. It's a nominal or ideal BC.

It's a different story when you actually fire a bullet and monitor its flight in terms of velocity decay. That tells you what your achieved BC was for that particular shot. Achieved BC would definitely be affected by RoT, and it could vary somewhat from shot to shot.

Just wanted to clear that up as it looked like one of those subtle details that get people thoroughly confused to the point of giving up.

yeah I looked at JDM but coulnt find itand a longitudinal cross section does not seem right.
but if you cut a cross section perpendicular to the long axis then you get a circle. which is what I was thinking as frontal area.
I assume the "skin" surface area is part of the drag coefficient.

oh just saw Dentons post thanks
iA is constant for caliber.

and the weight component clarifies a logical fault I have always had while thinking of BC as Cd..
namely that as Cd increases the projectile is less efficient, but BC is inversely related to the rate of velocity loss.

Mass and surface area have to be a oart of the solution. Imagine swinging a Japanese style fan around both extended and not. Same fan, but different surface areas....huge difference in wind resistance for the same mass.

The good news is that not very many can shoot into the needs of the second decimal point of BC.....Let alone the third.

Twist rate has nothing to do with BC. It does regulate gyroscopic stability factor, which is a double edged sword.

Sorry, I'm not following you. My statements were in regard to measurements conducted while actually firing and monitoring projectile flight. Some, with radar, were done with velocity decay. Others, with the Oehler equipment, were done with a velocity measurement and time of flight to a measured distance.

Interestingly enough, when I've participated in simultaneous measurements using radar and an Oehler, both systems gave substantially the same result.

My comments were more directed at what jwp said. Some who are not yet very familiar with all this might fail to realize the difference between an achieved BC and an ideal, published BC figure. I just wanted to illuminate the subject for tnem.

I do not know if I would call it an ideal BC as much as a theoretical BC.

The math formulas are an attempt to replicate what happens in the real world. They are not 100% accurate though.

We, as shooters would be much better served if the bullet manufacturers would publish real world (observed) BC.

I think Hornady is attempting to address this issue. We know they are at least testing and observing real world BC via doppler radar when they unexpectedly found the BC of their polymer tipped bullets diminishing in flight. This is what led to the development of their new polymer tip material advertised to not melt in flight.