Bullet density and Ballistic Coefficients
 

Now, we all know that a feather is going to drop to the ground slower than a lead ball because it's much less dense and is influenced by air much more. However, I'm wondering why there seems to be nothing to figure the difference in flight between, say, a plain copper bullet and a bullet with a lead core (let's ignore the fact that the copper bullets would have to have more volume to retain the same mass). The lead core one would have more density, and this would lead me to believe that it would be affected by wind less. However, ballistic coefficients are only calculated by shape, the relative angles of a certain bullet etc and this is what we use in calculators. What are standard projectiles made out of? Is compensation consistently accurate by adjusting BCs in calculators? I'm delving into the world of ballistics and some things just require good solid experience to answer :D

Only thing that affects drift is ability for a bullet to get from A-B. A bullet does not fight with the wind, like your example of a feather falling to the ground. But in flight it trys to keep even pressure on sides and flies with that. Where with gavity, it does not.

Gravity force and pressure from wind shear are two totally differnet things, and only things affecting it are bullets side area and its ability to get from A-B. Faster it gets there, less drift.

good explanation hits it right on the head it would have took me a page to explain it lol

Not to mention that a 50 gr. bullet weighs 50 grains weather it is 100% lead or 100% copper. The shape of that 50 gr. bullet needs to be concidered more so that what it's made of!
If you had 100 pounds of feathers compacted into a small ball like shape and 100 pounds of lead and drop them from the same height at the same time, they both should hit the ground at the same time, as long as the feathers don't have more wind resistance than the lead does.

Well, the problem I'm trying to figure out is that the feathers (and the copper) are not as dense as the lead. This would lead me to believe that they would be impacted by air more, both front-on (reducing velocity) and from the side (wind deflection). To have the same mass, there must be more volumetric displacement with a less-dense object. I'm trying to understand how this will change flight characteristics, and if there is a significance in this change noticeable at longer ranges. Also, if there is, I want to know what a standard projectile (G1, G7 etc) is made out of and how to compensate.

Not exactly true. The BC of a bullet can be ESTIMATED by looking at the form factor of the projectile, sectional density (the ratio of mass to cross sectional surface area) and the approximate velocity the bullet will be fired at, and comparing it to a similar bullet with an experimentally derived BC. Many bullet makers use calculations to approximate their bullets BC's because they know that a) most hunters never shoot far enough for BC to matter, but like the "bling factor" of a bullet with a high BC, b) they can get away with fudging the numbers to get a higher BC value than the bullet would have if actually tested, which sells more bullets (see "a") and c) it's a heck of a lot faster and cheaper to make an educated guess than to have someone actually carefully load and fire a few hundred rounds through chronographs at a variety of velocities and distances, then empirically calculate the BC based on real world bullet performance.

So what, exactly, is BC? Well, quite simply its the ratio of the velocity loss rate of a given projectile compared to a "standard projectile" fired at the same velocity. The G1 standard projectile is the one used by most manufacturers because it gives the highest BC numbers, but is also the LEAST representative drag model for modern spitzer boat-tail rifle bullets.

The G1 projectile


The caliber and shape of the bullet don't directly effect wind drift. The important factor is time of flight. If you compare a .308cal 180 grain bullet with a BC of .450 fired at 3000fps to a 100gr .243" VLD bullet with a .450 BC also fired at 3000fps, the two will have virtually IDENTICAL wind drift. Why? Because they are fired at the same velocity and lose that velocity at the same rate, so the time of flight to any given distance is the same.

Mike

Thanks mike,
However, I'm still not getting anything related to the flight characteristics of 2 different bullets made of materials with differing densities.

Here's a scenario that might help understand the question. Let's assume that we have 2 bullets. One made of lead, one made of copper. They are formed in the same mold, loaded in the same rifle and shot at the same speed. What is the difference? Surely there is something, beyond what a calculator using BC and velocity tells us.

If the bullets have the same shape the heavier bullet will have a higher BC. The formula is as follows:

BC=SD/Form Factor

BC is Ballistic Coefficient
SD is Sectional Density
Form Factor is a representation of the shape

Since the Form Factor is the same in your example the only variable is SD. So the BC is directly affected by the SD and if the SD goes up by 10% so does the BC go up by 10%.

I believe a lead bullet is 27% denser than copper so a lead bullet of the same shape will have a 27% higher BC.

Thanks Scott. That most definitely makes sense. Now a question for standard projectiles. Are standard projectiles assumed to be a certain density? Or are standard projectiles just a shape that form factor is derived from?

Is this correct? I thought BC was your friction coef (drag) in force vector analysis. I didn't know SD was part of it. You could be correct, just didn't know that.

Just did some research, guess your correct.