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.

Well, that equation is the one manufacturers use to ESTIMATE a bullets BC. The "form factor" variable is the one where manufacturers fudge. It would give a valid relative comparison between two bullets of identical shape, but the BC number won't necessarily conform to real world performance. You have to shoot the bullet to get an accurate MV and another accurate velocity reading downrange (at several distances if using the G1 standard), then, adjusting for atmospheric conditions, the average G1 BC can be calculated. If one uses the G7 standard with boattail bullets, the BC will remain much more constant over the entire velocity range, but the BC number will be smaller compared to the G1 BC. The G1 BC, when applied to bullets with a longer than 2 radius tangent ogive, secant ogive or boattail (which is most of the bullets fired from modern CF rifles), will change (decrease) as the velocity decreases. So a spitzer BT bullet at 3000 fps might have an actual G1 BC of .500, but at 2500 fps the BC may only be .450. At 2000 fps the BC might only be .375. If you use the .500 BC number (which is probably the number published by the manufacturer) in a ballistics calculator to calculate the elevation for a long range shot, you would likely miss low because the G1 BC doesn't remain constant as the bullet loses velocity. The G7 standard is much better for most rifle bullets because the drag model closely matches the G7 standard bullet, meaning that the BC changes very little if at all over the entire flight of the bullet. But manufacturers are reluctant to switch to it because the G7 BC number itself is far smaller than the G1 BC (for example, a Berger .308cal 185gr VLD has a G1 BC of 0.549, but a G7 BC of 0.281, but the G7 BC will give much more accurate to real world results when used with a ballistics program like JBM).

Mike

The best way to explain why in simple terms is that "An object in motion tends to stay in motion unless acted upon by an external force". For a bullet the external force is air resistance. For both the copper and lead bullet of the same shape the air resistance is actually the same.

The reason the lead bullet carries its velocity better is that its additional weight makes it harder too slow down due to higher FT/LBS momentum. So more air resistance is required to stop the object of greater mass.

Your question is flawed. This is an impossibility. If two bullets are made of solid copper and solid lead, while being the exact same shape and size, the lead bullet will weigh more. That obviously means it won't be starting as fast with the same charge of powder behind it.

And if you want to switch it around so that the two bullets weigh the same, then their shape will be different. The copper bullet would be elongated and more aerodynamic, unless of course you wanted to make it a flat based round nose and have the lead bullet be a boat tailed spitzer...

JM,
I understand what you're saying. However, even if we elongated the copper bullet to retain the same mass it's BC suffers. Have you seen the Barnes MRX? It has a dense alloy in the rear of the bullet to give it additional penetration, and a higher BC compared to other plain-copper barnes bullets (it's expensive though :D).

In the lighter for caliber bullets the copper bullets will have more potential for a higher BC. Take for example the ETip 150 grain with .469BC and 168 grain with .503BC. They are very long and sleek for their weight and have an advantage in Form Factor.

However as the bullets get heavier the copper bullets loose their advantage in form factor because they must be made to be stablilized in standard twist barrels and the bullet length must be limited. This results in a more rounded nose section in order to add mass in the length allowed in a given twist rate. The Form Factor suffers slightly from the relatively blunt nose.

The reason Barnes adds the dense material is to maintain the sleek nose section and still be within a length that can be stabilized. So in heavy bullets 200 - 240 grain 30 cal lead will actually have an advantage in BC potential.

SG,

I thought I read somewhere that the length and toughness of the copper led to the requirement of the "triple-shock" bands to reduce bering surface and pressure.

Anyways, let's pretend we have a copper and lead bullet of the same mass. The stabilization and velocity are not limiting factors in our pretend setup. They are both stabilized adequately and are shot at the same speed. They are both of a similar aerodynamic shape, but the copper bullet is obviously longer and sleeker to maintain the equal mass. Under what conditions can we assume that one bullet will be better than the other? Will the lead, with its greater density always have the higher BC? Will its density make up for its less-sleek shape?

If the length of the bullets is not limited the copper bullet will always have a higher potential BC.

This is the case with lighter for caliber bullets.

With the heavy for caliber bullets stabilization requirements will limit length and lead will actually have an advantage. So the answer to which is better depends on the application.

Thanks SG,

You've ultimately summarized it: The conditions are not necessarily set, although generally copper bullets have more potential in light-mid weights and those that are primarily concentrated of lead would be the choice at heavier weights. And also, if two bullets are shaped the exact same, the denser one will have the higher BC. Another file is in the brain.

"As a rule" I am sceptical of things that sound to good to be true. I looked at the 155 Scenar bullet a while back because its claimed BC.508 was so high relative other bullets in its weight range.

As one would expect when actual measured numbers were compared to the claimed numbers by knowledgeable shooters the claimed BC was significantly overstated. Measured BC came in at around .459. Keep in mind that the Scenar is a match bullet and its construction is not limited by the integrity demands of the hunting bullet you are comparing it to.

That big hollow area behind the point is just a great engineering solution to overcome the density disadvantage of lead in the lighter bullets without the cost of copper.

If you are comparing match bullets to hunting bullets it is apples and oranges but even at that the lead bullets come up short to the new copper hunting bullets from Nosler.

.505 is their claimed and actual BC,though of course atmosphere and temp particulars,do change all BC's every day. The 155 Scenar's supeerklative BC is easy to glean from even a casual glimpse as it's length/profile readily concur. Set it next to 168 or 175SMK's,for conversation. It smokes the newly reconfigured 155 Palma SMK as well. I shoot the Scenar by the jazillion and my elevation erector's have no bias built in.

As an aside,Scenar terminal affects are far more reliable than the oft touted SMK,espoused by the Mall Ninja Faction. The Scenar's faux nosecone,is a simplistic and effective approach to increasing projectile length,while adding a favorable BC profiling to a given weight.

I much prefer V-Max and A-Max upon Critters,to the oft touted NBT. Prefer Scenar's to them as well.

Lapua BC's are spot on,as a minimum.........................

Rifle Looney, I mean Relentless Pursuit, I mean Trump Card with all due respect I will take Brian Litz evaluation of BC below, along with the evaluations of top competative shooters to your opinion based on the manufacturers claimed BC only.

http://www.appliedballisticsllc.com/...pua_Scenar.pdf

As I mentioned earlier when a bullet BC is that far from the norm one should take those claims with a grain of salt.

Actually, this isn't looney. Looney is bill Wilds from NY. This is Larry, from laska.