Fat ain't where it's at...it's BC...


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Jasper1573
January 17, 2013, 06:32 PM
There is no replacement for displacement! Thin may be in...but fat's were it's at!

The weight of the projectile wins, between the two.


I was reading another thread where the above statements were made. The thread went a bit sour, so I opted not to post there.

Projectile weight doesn't determine how well a bullet performs at long range...it is the ballistic coefficient (BC) of the bullet.

I realize that weight and bullet length and caliber/diameter play into BC, but in the end it is the BC that determines a bullet's ballistic performance.

Example: I have a Rem 700 in .308 Win and another in .243 Win. I shoot a 175 grain bullet from the 308 and a 95 grain bullet from the 243. While the BCs of these two bullets are about .02 different (.496 vs .480), they are close enough, given the same or very close velocity, that I can use the same ballistics out to 1000 yards (bullet drop and wind deflection) for both and be within a few inches, even though there is an 80 grain difference between the two.

If fat were really where it's at, given the same velocity, a 230 grain 45 ACP bullet with a .250 BC would perform better than a 175 grain .308 Win bullet with a BC of .496...it ain't so.

If we are talking about knock down power, then I have to give the nod to the heavier bullet (within reason).

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MErl
January 17, 2013, 06:37 PM
is this true when you get up to the really big things like 30mm as well?

cal30_sniper
January 17, 2013, 06:47 PM
Ballistic Coefficient does nothing more than determine how rapidly a bullet will slow down in flight. The higher ballistic coefficient, the less drag, meaning that it retains velocity longer. A bullet with higher drag and less ballistic coefficient slows down faster.

Bullet weight has nothing to do with velocity once it has left the barrel. It takes more energy to get a heavier bullet to the same speed as a lighter bullet, hence more powder and a heavier recoil. However, once they are in flight, weight has no impact on velocity. This is due to the same principle as a heavier object falling the same speed as a lighter object. Gravity does not care how heavy an object is. It all falls at the same speed. The reason a higher ballistic coefficient bullet drops less, is because it retains velocity longer, it reaches the target faster. The longer it is in the air, the more it falls.

Once the bullet reaches the target, ballistic coefficient has done its job. It has no real impact on "terminal performance". That is controlled by caliber and weight, which when combined, yield the term sectional density. Similarly, the amount of energy input to the target is determined by velocity and weight. Bullet type and design will determine the rate of expansion, fragmentation, etc, and that will fill out the picture of "terminal performance."

Hope that helps simplify things.

Jasper1573
January 17, 2013, 06:47 PM
I have no personal experience with such large projectiles...30mm is generally fired only from a mini-gun, in my military experience, from an A-10 aircraft at tanks and such. But if science and physics be true and consistent, and they are, then I suspect that the rules of ballistiics apply regardless of the projectile caliber.

Jasper1573
January 17, 2013, 06:51 PM
terminal performance

You are correct...I used the wrong term. I should have said ballistic performance over long ranges...original post will be edited to reflect this.

Thanks,

Jasper

cal30_sniper
January 17, 2013, 06:52 PM
One added note for in flight performance. Bullet weight (more properly its mass) will affect how much the bullet is influenced in flight by wind or other forces imparted on it. The ballistic coefficient will also influence how much it is affected by aerodynamic forces other than straight drag. However, consider two bullets of equal coefficient, but one is lighter than the other. The lighter bullet will be affected more by external forces such as wind than the heavier bullet. The force acting on the bullets is the same, hence the lighter bullet will be accelerated faster in the direction of the force. Remember, Force = Mass * Acceleration, or Acceleration = Force/Mass. The heavier bullet will have less acceleration, thus it will be less affected by external forces in flight. By definition, both bullets will be equally affected by drag due to their identical ballistic coefficient.

Walkalong
January 17, 2013, 06:55 PM
The hunter tends to see performance as on target results, where the long range target shooter tends to see performance as less bullet drift/drop.

The OP is talking about the latter.

Jasper1573
January 17, 2013, 07:06 PM
consider two bullets of equal coefficient, but one is lighter than the other. The lighter bullet will be affected more by external forces such as wind than the heavier bullet.

I don't believe this to be true...when I run a ballistics calculator on the two bullets mentioned above, they have exactly the same drop and wind deflection.

Walkalong
January 17, 2013, 07:09 PM
For a lighter and heavier bullet starting at the same speed both with the same BC it would be true.

cal30_sniper
January 17, 2013, 07:15 PM
I stand corrected. After a bit of research, it appears that the formula for determining Ballistic Coefficient includes not only the coefficient of drag, but also the mass of the projectile. As such, they will fly exactly the same as long as they start at the same velocity. And require no off angle forces. Same basic explanation as above, just a more correct way of explaining it.

Forgive me, I'm a lot more familiar with aerodynamics than ballistics.

I'm still not entirely sold on transverse wind gusts or things such as raindrops and dust though. I would argue that a lighter bullet is still going to be affected more by changes of force that it encounters in flight. The difference should be minute because of the angles involved, but it will be there. Especially when encountering liquid or solid objects.

So yes, in a ballistic program they will fly the same. In real world, they may not.

cal30_sniper
January 17, 2013, 07:29 PM
Let me try to answer the question this way. Your ballistics program assumes a constant crosswind. Since both bullets have the same ballistic coefficient and started at the same velocity, they will both reach the target at the same time. Because they are in the air for the same amount of time, the ballistics program calculates that they will have the same amount of wind drift. That's not entirely true. Depending on the exact shape of the bullet, and the angle at which it is encountering the wind, one may be more or less affected by the crosswind. Ballistic coefficient is calculated for a bullet flying straight through the air, not one experiencing an off-angle wind. It would take a very complex CFD program to get the actual result that an off angle wind would actually have on each projectile.

Similarly, the effect of a wind gust or foreign particle in the air would also be impacted by the shape of both the bullet, and angle of gust or shape/angle of particle impact. All things considered equal, the lighter bullet would be more affected than the heavier bullet by such forces.

Remember, ballistic coefficient is calculated for air approaching head on, not at an angle. Ballistic coefficient is also a function of velocity, something else your ballistics program does not take into account. Different shapes will have different rates of change in ballistic coefficient as a function of velocity. All of this creates differences between two bullets of different weights that have the same advertised "ballistic coefficient".

jmr40
January 17, 2013, 07:34 PM
If we are talking about knock down power, then I have to give the nod to the heavier bullet (within reason).


You are basing your conclusions on how a bullet flies and mathematical energy numbers

You are forgetting bullet construction. Bullet construction determines what happens "AFTER" the bullet impacts. BC controls what happpens before impact. There are 2 schools of thought. I think both work equally well as long as they are applied correcly.

One school of thought calls for a bullet that expands rapidly and causes massive internal injuries. You need a bit more bullet weight to get adequate penetration because the bullets expand so fast you lose lots of bullet weight. As long as you can hit vital organs from an angle that does not require a lot of penetration this will put game down the fastest. This works well at longer range with the heavier bullets with good BC's. The Berger bullets are a good example. They also expand well at slower speeds which helps at long range.

The Barnes bullets are an example of the other extreme. Using hard, lightweight bullets moving very fast. While a 168 Berger bullet has much more energy, I can shoot a 130 gr Barnes bullet about 400 fps faster at the muzzle from my 308. The explosive Berger will lose more than 50% of its weight on impact and end up weighing under 100 grains. The 130 will hit much faster, retain close to 100% of its weight. Even though it has far less energy, it will still shoot flatter, and penetrate much deeper. At closer ranges a 130 Barnes would be a better choice on larger, tougher game than a 168 Berger where penetration is needed.

This holds true at normal hunting ranges. At extreme ranges, 400+, the nod certainly goes to the heavier, more explosive bullet. If the hard bullets impact too slow, you don't get any expansion. But not many shooters are good enough to shoot at game that far away.

Jasper1573
January 17, 2013, 07:50 PM
Remember, ballistic coefficient is calculated for air approaching head on, not at an angle. Ballistic coefficient is also a function of velocity, something else your ballistics program does not take into account.

1. I don't know if BC is calculated with a headwind or not, but my ballistics program does take into account wind at an angle...it assumes a 90 degree angle for max deflection. If the angle is greater or less, then the shooter must judge the effect. I suspect there are programs that will calculate wind deflection at various angles, but mine doesn't.

2. The ballistics program I use and every other one I have seen require velocity as an input...bullets have different performance based on the bullet velocity.

cal30_sniper
January 17, 2013, 08:05 PM
The ballistics program requires velocity so it can calculate a trajectory. You couldn't even begin to calculate the trajectory without knowing the muzzle velocity. Let me try to break this down simpler.

Trajectory, i.e. bullet drop, is determined ONLY by how long the bullet is in the air. All objects fall at the same speed, no matter what their weight. From the moment the bullet leaves the barrel and ceases to have a supporting force applied to keep it up, it begins to fall.

The only way to calculate how long the bullet is in the air is to know the starting velocity, and then be able to calculate how quickly it slows down. Ballistic coefficient is a measure a bullets ability to overcome air resistance in straight flight. It is a function of mass, diameter (cross sectional area), and coefficient of drag. Mass is constant. The cross sectional area is not. When the bullet is traveling straight through the air, the diameter is used to describe its cross sectional area. However, if it is moving through the air at an angle (think tumbling or cross wind), the cross sectional area is changed. It may be close, but it will be changed. Finally, the Coefficient of drag is also a function of velocity. As velocity changes, so does the C_D. That's why bullet manufactures like Sierra include multiple BCs for each bullet, tabled by velocity. Since that velocity changes throughout the bullets flight, so does the C_D. Therefore, so does the BC.

I think you can see now that the actual problem is much, much, MUCH more complicated than the simple trig and physics problem that is calculated by your ballistics program. That ballistics program will get you close, but the further out you shoot, the further the actual trajectory is going to diverge from the calculated one. Just like everything else in science and engineering, it is an approximation of a simplified problem. It works fine for estimation, but it is not a complete solution.

RPRNY
January 17, 2013, 08:09 PM
Jesuitical
[jezh-oo-it-i-kuh l, jez-oo-, jez-yoo-] Show IPA
adjective
1.
of or pertaining to Jesuits or Jesuitism.
2.
( often lowercase ) practicing casuistry or equivocation; using subtle or oversubtle reasoning; crafty; sly; intriguing

How many angels fit on a BC?


Okay, just kidding, but these types of discussions are so academic and theoretical. There's no "one thing". Big and slow makes big holes in critters but not at great distances. Small and fast may make very predictable and repeatable holes in paper at mid-range but may need to be bigger to do the same at longer distance. The 50 BMG is a proven master of long range target shooting and killing, but not highly recommended for prairie dogs nonetheless. Der Zuperscreaminloudenboomer may reach 63% the speed of light, but one would not be advised to use it on dangerous game.

Use the best combination of bullet weight, diameter, and construction aligned to the velocity required for optimal terminal performance that you can comfortably and reliably shoot. That's where it's at ;-)

cal30_sniper
January 17, 2013, 08:16 PM
I'm confused. Is the OP asking about terminal performance, as in how the bullet does after it strikes the target, or are we talking about ballistic performance, as in what happens on the way to the target?

Two completely different and almost completely unrelated subjects.

Jasper1573
January 17, 2013, 08:20 PM
It may be close, but it will be changed. Finally, the Coefficient of drag is also a function of velocity. As velocity changes, so does the C_D. That's why bullet manufactures like Sierra include multiple BCs for each bullet, tabled by velocity. Since that velocity changes throughout the bullets flight, so does the C_D. Therefore, so does the BC.

I think you can see now that the actual problem is much, much, MUCH more complicated than the simple trig and physics problem that is calculated by your ballistics program. That ballistics program will get you close, but the further out you shoot, the further the actual trajectory is going to diverge from the calculated one. Just like everything else in science and engineering, it is an approximation

Yep, I agree with your above statements...but if I can shoot two bullets, the ones mentioned in my original post, and get very similar results based on very close BCs at very similar velocities, then I try to apply the KISS principle...things are complicated enough without having to think too much and make my brain cramp.

Greatly appreciate the discussion...haven't thought about coefficient of drag and such since I was navigating C-130s and doing air drop...good memories. You should have given Galileo a footnote;-)

cal30_sniper
January 17, 2013, 08:26 PM
Haha, just don't screw it up like Aristotle did.

As far as the lighter vs heavier thing though, the difference comes after the bullets hit the target. If you're punching paper, there is no difference. If you're killing things, there's a big difference.

Assuming the same muzzle velocity and the same BC, they reach the target at the same velocity. The heavier bullet is then going to deliver much more energy to the target. Everything that has been said so far about light and fast bullets is true, but a heavier bullet has all the advantages if they strike at the same velocity. The only way you can get ahead with a lighter bullet is if you can start it significantly faster, or get it to hold more velocity through a better BC.

Thanks for your service. I'm on my way to flight school for the Marine Corps as soon as I finish up with my Masters degree here. Something tells me I'm not quite done studying aerodynamics yet...

Jasper1573
January 17, 2013, 08:33 PM
How many angels fit on a BC?

That's a good one!

I'm on my way to flight school for the Marine Corps as soon as I finish up with my Masters degree here. Something tells me I'm not quite done studying aerodynamics yet...

Once you are out there flying, keep it straight and level unless the need arises to do otherwise. Be safe and stay alive...our country needs patriots.

Finally, don't do anything "dumb, dangerous, or different." We generally used to end every flight briefing with those words because violation of the 3 Ds can be deadly.

God bless and protect America!

Abel
January 17, 2013, 09:34 PM
uhh....

...30-06!

yeah.

conrad427
January 17, 2013, 09:40 PM
My thoughts exactly, Abel!

TNBilly
January 18, 2013, 12:26 AM
Jasper, here's one for ya. Take two same design bullets of fairly close length, closer the better but of differing calibers of course. The difference in the BC will be that part attributable to the weight difference.
and yes weight does have a trade off point where the higher BC pays off in velocity retained better over longer distance. Simple self evidence would be were it not, everyone would be running the lightest bullets in a long range class instead of the heavier most efficient ones in the caliber.

helotaxi
January 18, 2013, 08:55 AM
One added note for in flight performance. Bullet weight (more properly its mass) will affect how much the bullet is influenced in flight by wind or other forces imparted on it. The ballistic coefficient will also influence how much it is affected by aerodynamic forces other than straight drag. However, consider two bullets of equal coefficient, but one is lighter than the other. The lighter bullet will be affected more by external forces such as wind than the heavier bullet. The force acting on the bullets is the same, hence the lighter bullet will be accelerated faster in the direction of the force. Remember, Force = Mass * Acceleration, or Acceleration = Force/Mass. The heavier bullet will have less acceleration, thus it will be less affected by external forces in flight. By definition, both bullets will be equally affected by drag due to their identical ballistic coefficient.Completely incorrect. BC is all that matters when calculating external ballistics. The mass of the bullet divided by the square of the caliber determines sectional density and sectional density factors into the ballistic coefficient. As such, all that matters about the bullet mass (it proportion to the frontal area of the bullet) is captured in the BC. Wind deflection is a drag function. BC is the drag model of the bullet. Wind deflection is modeled with BC.

The flaw is thinking is that the BC determines the drag force on the bullet. The BC determines the drag acceleration on the bullet already normalized for bullet mass. As such, weight doesn't matter; if you have the BC, you have all you need to know about how the bullet will fly.

If you want to run a little "look see" run two bullets of different calibers and weights with the same BC through a ballistic calculator using the same MV. The ballistics will be identical, to include wind drift.

helotaxi
January 18, 2013, 09:06 AM
Remember, ballistic coefficient is calculated for air approaching head on, not at an angle.Air is always approaching head on. The bullet weathervanes into the wind. The result is that the combination of drag from the bullet's velocity and the wind drag pushing it to the side act along the axis of the bullet on the same point and form a single resultant drag vector determined by the ballistic coefficient of the bullet and its velocity. Ballistic coefficient is also a function of velocity, something else your ballistics program does not take into account.This is only kind of true. Some ballistic programs allow for a progressive BC. But on the whole it isn't nearly as important as it seems if you have a properly averaged BC over the velocity range that you're actually using the bullet and not a single velocity BC. Bryan Litz goes over this in detail in Applied Ballistics for Long Range Shooting. You should really pick that one up and give it a read. I think that you'll find it addresses all the common misconceptions.

As far as hitting solid or liquid...at that point all bets are off and you're more likely to experience a dynamic upset, regardless of mass. For ultralight particles like dust, the shockwave from the bullet makes sure that they never touch it.

cal30_sniper
January 18, 2013, 10:42 AM
Completely incorrect. BC is all that matters when calculating external ballistics. The mass of the bullet divided by the square of the caliber determines sectional density and sectional density factors into the ballistic coefficient. As such, all that matters about the bullet mass (it proportion to the frontal area of the bullet) is captured in the BC. Wind deflection is a drag function. BC is the drag model of the bullet. Wind deflection is modeled with BC.

The flaw is thinking is that the BC determines the drag force on the bullet. The BC determines the drag acceleration on the bullet already normalized for bullet mass. As such, weight doesn't matter; if you have the BC, you have all you need to know about how the bullet will fly.

If you want to run a little "look see" run two bullets of different calibers and weights with the same BC through a ballistic calculator using the same MV. The ballistics will be identical, to include wind drift.

As was stated before, a ballistics program uses a simplified trig and physics problem to approximate what's actually going on, under what would be considered "ideal" conditions. You running two different bullets through the same calculator and coming out with the same answer shows nothing more than your program can work the math correctly.

When wind is calculated into that program, it assumes a steady wind from a constant direction. You won't often find that in the real world. Any change in wind or gust is going to buffet the bullet in flight. A heavier bullet will be affected less. It's simple physics.

Again, the very definition of BC shows that it will not usually remain the same for a heavier and lighter bullet. Since the calculation is done with mass and diameter (which remain the same) and drag coefficient (which is a function of velocity), it will change throughout flight. The ONLY way that a heavy and light bullet will fly the same trajectory is if they have exactly the same sectional density, and their drag coefficient changes exactly the same amount as the velocity decreases (or if they have different sectional densities, but the drag coefficient changes at a set ratio which offsets their difference in sectional densities). Obviously, both of those cases are aerodynamic flukes, and a pair that actually does that would be extremely rare. Here's some ratios on that effect:

Two bullets have the same ballistic coefficient at an identical velocity. The sectional density of bullet two is twice as great as that of bullet one. They experience the same change in drag coefficient as velocity drops. The ballistic coefficient of the bullet with the smaller sectional density is affected twice as much as that of the larger SD bullet. The smaller SD bullet is now losing velocity faster than the higher SD bullet.

You are correct about bullets turning into the wind. However, that only takes into account a steady state wind condition. It does not take into account varying wind and gusts throughout the bullet flight. Even though ballistic coefficient takes into affect drag acceleration, it does not take into account center of gravity and moment of inertia. Both of which will determine how fast a bullet can "turn into the wind". The mass and shape of the bullet are the only things that determine CG and moment of inertia. Therefore, they will in some way determine how much the bullet is affected during the time that it faces lateral forces before it can turn into the wind.

Again, ballistics programs are a very good estimate. However, there are numerous reasons why they are not completely accurate in the real world. When you leave the realms of simplified trajectory programs and enter the atmosphere, bullet weight will have an impact, albeit a small one.

murf
January 18, 2013, 01:45 PM
cal30 sniper,

a lighter bullet will have less surface area affected by the wind than a heavier bullet of the same bc.

wind, no matter which direction or velocity, will have the same effect on bullets with the same ballistic coefficient, regardless.

murf

Searcher4851
January 18, 2013, 04:20 PM
I'm no expert on this stuff by an means, but doesn't a heavier bullet experience less wind deflection due to inertia?

murf
January 18, 2013, 06:09 PM
searcher4851,

if the bullets had the same surface area, sure. but bullets with the same bc don't. the heavier bullet will have the greater surface area.

kind of like a bigger sailboat having bigger sails so it can go just as fast as the smaller boat.

murf

cal30_sniper
January 18, 2013, 07:52 PM
A "heavier bullet" can easily go just as fast or faster than a "lighter bullet." It all depends on what cartridge you're using. A 300 Win Mag is going to launch a much heavier bullet faster than a much lighter bullet shot out of a .243. What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?

cal30_sniper
January 18, 2013, 08:10 PM
cal30 sniper,

a lighter bullet will have less surface area affected by the wind than a heavier bullet of the same bc.

wind, no matter which direction or velocity, will have the same effect on bullets with the same ballistic coefficient, regardless.

murf

I'm still struggling with this statement in my mind. I'm going to have to go mull it over for a bit...

Initial thoughts though. Surface area does not always have the same correlation with weight for a ballistic coefficient. Surface area from directly head on is directly proportional to diameter of the bullet squared. However, from any other angle, the shape of the bullet, and its density, are going to change its surface area relative to its mass or sectional density. I'd say that in any situation I can think of, the heavier bullet should have a larger area than a lighter bullet, but it definitely wouldn't be a 1:1 ratio.

Also, since bullets can be filled with varying substances and covered with different jackets, tipped with different polymers, or left hollow in the nose, a bullet of the same weight and caliber (thus the same sectional density), with the same coefficient of drag, could have the same ballistic coefficient as another bullet but a different shape profile. That would result in differing surface areas experiencing air for anything other than flight with a zero-value wind. The different surface areas would cause the ballistic coefficient to change at varying rates for the two bullets.

However, a BC is calculated for air going straight over the bullet, hence why the diameter of the bullet squared is used as the critical area. At any other angle other than directly forward, the BC is going to change. Two bullets may have the same sectional density and coefficient of drag from dead on, but if they angle sideways and aren't exactly the same shape, that would change, if only slightly, correct?



Thoughts?

Auto426
January 18, 2013, 08:55 PM
What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?

I think that's what the OP is trying to argue, but it seems to me that's not what the post the OP quoted was trying to argue. The quoted post seems to be more along the lines of bigger bullets make bigger holes, not so much about how they fly through the air.

Then again I didn't read the thread that post came from, so I don't really know.

helotaxi
January 19, 2013, 08:17 AM
What is being discussed here, is if the .243 and .300 bullet were the same BC, and started at the same velocity, would they always arrive in exactly the same place?According to Bryan Litz, who is an aerospace engineer, a very accomplish long range shooter and the chief ballistic guru for Berger Bullets, the answer is "yes".

It doesn't matter if you're talking gusts, steady wind, eddies, or whatever, the BC determines how the bullet reacts. Especially when you get into long rage bullets better described by the G7 model, the variability with velocity is accounted for very well. In fact the G7 model accounts for the change in drag coefficient with velocity as part of the model. Bullet mass doesn't play a part in the variability at all.

As far as the side area of the bullet, it is totally irrelevant. The bullet will weathervane into the wind, even with transient gusts. You have to consider that the amount we're talking about here is fractions of a degree since the amount that it yaws is determined by the ratio of the cross wind component to the bullet velocity. So for a bullet traveling at 2000fps and experiencing a 20mph crosswind, the yaw angle would be the arctangent of the crosswind/velocity or 0.8 degrees. When the wind gusts, that yaw angle changes a bit to continue to nose into the resultant wind from the combination of wind vector and velocity vector.

Because the bullet weathervanes, only the frontal area and frontal drag profile matter. Those are described by the BC. Sectional density captures the mass as it relates to that frontal area and the form factor captures the drag as it relates to that frontal area. The result, along with velocity gives you total drag acceleration normalized for bullet mass. In effect mass is cancelled out of the equation because of how the trajectory is modeled. Does bullet mass matter? Yes, but the way and the degree to which is matters for all external ballistics calculations, all, is captured in the BC.

wanderinwalker
January 19, 2013, 09:13 AM
It's really, really simple, really. Two bullets of the same BC, launched at the same speed, have the SAME downrange drop and wind dift values. Bullet weight and diameter has NOTHING to do with it.

Don't overthink and overanalyze it. Get out and shoot some stuff at long range, beyond 300 yards and begin to understand what happens in flight. And if you're shooting long range paper targets and are concerned about wind, get yourself a nice hot rod 6.5mm and toss 142gr bullets.

helotaxi
January 19, 2013, 09:16 AM
When wind is calculated into that program, it assumes a steady wind from a constant direction. You won't often find that in the real world. Any change in wind or gust is going to buffet the bullet in flight. A heavier bullet will be affected less. It's simple physics.Ever seen a large semi get blown off the road by a strong wind gust? What about a relatively lightweight sports car? You're right it is physics, and it's pretty simple when you look at the correct variables. Weight isn't one that you look at by itself here. Just like the semi, weight alone doesn't overcome wind. You have to look at sectional density because that is weight as it actually applies here, which is in proportion to area which is what determines the drag force.

You are correct about bullets turning into the wind. However, that only takes into account a steady state wind condition. It does not take into account varying wind and gusts throughout the bullet flight. Even though ballistic coefficient takes into affect drag acceleration, it does not take into account center of gravity and moment of inertia. Both of which will determine how fast a bullet can "turn into the wind". The mass and shape of the bullet are the only things that determine CG and moment of inertia. Therefore, they will in some way determine how much the bullet is affected during the time that it faces lateral forces before it can turn into the wind.With a long range bullet design, the CoG and the CoP are not conicident with the CoP being forward of the CoG. The result is a shape that is unstable unless spun. It's actually the spin that both allows stability and forces the nose into the wind aligning the CoG and CoP on the same axis and keeping the forces acting on them from causing the bullet to tumble. Bullet wieght doesn't really play into this at all. Wind transients are not digital transitions, they are analog. The bullet doesn't have to instantaneously snap from 0.2 to 1.0 degree of yaw, there is a gradient. We're also talking about miniscule amounts of change. The overall result has been shown to be negligible both in theory and in practice.

Again, ballistics programs are a very good estimate. However, there are numerous reasons why they are not completely accurate in the real world. When you leave the realms of simplified trajectory programs and enter the atmosphere, bullet weight will have an impact, albeit a small one.Modern ballistics programs, even ones that you can run on your smartphone are far from simple. The good ones actually allow you to program in wind variability along the bullet's flight path. In the case of those using the G7 BC, velocity effect on drag is accounted for as well. The simplicity or lack thereof of what you and I have access to, however, does not factor in to what actually has been measured and shown to be the case by people with a very advanced understanding of the forces involved, what matters and what doesn't and what the overall results are. This includes the DoD with access to radar sophisticated enough to track a projectile as it travels downrange to engineers with access to supersonic wind tunnels and the ability to program 6 degree of freedom simulations including all the variables and then test the simulations against real world results. Their findings are that mass, in an of itself, isn't important when determining how a bullet flies. BC tells you all that you need to know. The models also account for the variability of drag with velocity though the G7 model does a better job of this on average with a single number than the G1.

murf
January 19, 2013, 12:46 PM
wanderinwalker,

bullet weight and diameter have everything to do with this conversation. bc is equal to the sectional density of the bullet divided by its form factor. sectional density is equal to the MASS (weight) of the bullet divided by its DIAMETER squared.

heliotax,

your weathervane equation does not include bc, so yaw angle would have nothing to do with the shape of the nose of the bullet. it has everything to do with force applied in two different directions. if the crosswind was blowing at 2000 fps, the bullet would yaw at a 45 degree angle, regardless of nose shape.

murf

murf
January 19, 2013, 01:11 PM
cal30 sniper,

surface area is a square function. bc is a square function (diameter squared). force applied by the wind is a square function (mass times acceleration). i think surface area and bc are directly proportional in this case.

but, i'm no scientist.

murf

wanderinwalker
January 19, 2013, 01:13 PM
wanderinwalker,

bullet weight and diameter have everything to do with this conversation. bc is equal to the sectional density of the bullet divided by its form factor. sectional density is equal to the MASS (weight) of the bullet divided by its DIAMETER squared.


Which I am aware of, thank you, actually. But once you have BC (let's say .5), two bullets of .5 BC will fly the same at the same velocity. Doesn't matter if it's a 123gr 6.5mm, 107gr 6mm or 175gr .308", they will have the same drop and drift when launched at 2700-fps.

Going to a bigger bullet just for "more mass" thinking it will improve your wind resistance doesn't work. Say going to a standard 180gr .308" soft point hunting bullet versus a 155gr Palma bullet. The 155 is going to fly better (and has the advantange of being able to be pushed faster).

I was pretty sure after reading the OP, I was in agreement with his statement here:
Projectile weight doesn't determine how well a bullet performs at long range...it is the ballistic coefficient (BC) of the bullet.

I realize that weight and bullet length and caliber/diameter play into BC, but in the end it is the BC that determines a bullet's ballistic performance.

This of course is ignoring terminal ballistics, which tends to favor bigger bullets, all else being equal.

At the end of the day, what counts most is actually tossing the bullets in question down the tube and seeing what you get down range. I will happily shoot my 80gr .223 600-yard ammo all day against guys running 168gr .308s and not feel any disadvantage. But a 142gr 6.5mm slug trumps both, in spite of being lighter than the .308" bullet (it's longer and more efficient aerodynamically). And FWIW, the 123gr 6.5mm match bullets have pretty good BCs too.

From what I have seen on the firing line at matches, the winning combination is the best bullet that can be launched with tolerable recoil. If "the best" was the biggest and baddest, everybody would be using .300 Magnums with 220gr Matchkings or .338s with 250gr. Yet that's not what I see on the line...

d2wing
January 19, 2013, 01:26 PM
Which is why Sniper use larger bullets for ranges in excess of 1000 meters. What is true at 600 meters is not true at 1600 meters for example. The greater the mass the greater the potential B.C. At smaller calibers you cannot have enough mass to have optimal overall ballistics including total energy to overcome drag at long range.

murf
January 19, 2013, 01:35 PM
wanderinwalker,

agreed. 50 bmg would rule.

the op stated a fact: bullets of equal bc will have the same trajectory and the same wind drift. i'm not arguing this fact.

apparently, he is having a hard time understanding "why". i'm just trying to explain things in laymans terms and analogies to try and help him.

if this is a case of "the blind leading the blind", hopefully, someone else will chime in.

murf

helotaxi
January 19, 2013, 01:42 PM
your weathervane equation does not include bc, so yaw angle would have nothing to do with the shape of the nose of the bullet. it has everything to do with force applied in two different directions. if the crosswind was blowing at 2000 fps, the bullet would yaw at a 45 degree angle, regardless of nose shape.
Correct. BC does not affect whether or not the bullet yaws into the wind, it does so the same amount regardless of the BC based solely on the relationship of the crosswind component to the bullet's velocity. I never claimed anything to the contrary. But the angle of yaw is not what determines wind drift, merely what explains it and explains why the BC is what matters in determining the degree of wind drift. Wind effects are a drag function just like velocity decay and the yawing of the bullet into the wind is what keeps the cumulative drag effects of wind and velocity acting inline with the bullet's longitudinal axis.

wanderinwalker
January 19, 2013, 01:46 PM
Ah, that must be the part missed. We are talking the same thing in reality. All I can suggest is maybe the best way to clarify it all is a visual demonstration. Go to the range where different combinations are being used and see what is happening. Sit behind a shooter and scope the bullet path through the mirage in a humid day.

Ironically I was thinking about the 50 BMG after my last post. It may be the king of shoulder fired long range arms but I've read how in WWII it lacked range for aerial combat vs the larger 20mm guns. I guess my point is "there is always something better."

cal30_sniper
January 19, 2013, 01:55 PM
Thanks for the lesson guys. I was still confusing ballistic coefficient and drag coefficient. It all makes sense now, and I completely agree that the heavier bullet and lighter bullet will fly exactly the same.

As far as the aerial guns, I believe the difference was mostly that the 20mm shell had an explosive projectile, while the .50 used was normal AP. The problem with 20mm shells was they took up so much space, so firing length was very short before they ran out of ammo. Cannon shells have actually been used as shoulder fired weapons before though. The communists used 20mm anti-tank rifles as "sniper rifles" during Korea. I always thought that was interesting.

d2wing
January 19, 2013, 02:13 PM
You also take into consideration surface to mass of the same shape and time vector of force.

helotaxi
January 19, 2013, 02:24 PM
What is true at 600 meters is not true at 1600 meters for example.Not sure what you mean by this. Physics don't change with range.The greater the mass the greater the potential B.C. At smaller calibers you cannot have enough mass to have optimal overall ballistics including total energy to overcome drag at long range.Mass only matters as it relates to cross sectional area. You could have a very efficient small caliber bullet if you made it extremely long or of something extremely dense both which would up the sectional density. Where you would run into problems with such a design is propelling it at a high enough velocity to realize the advantage of the increase in BC. Mass alone doesn't mean anything. A 120gn .30 cal bullet has no more potential for a high BC than a 68gn .224cal bullet even though it weighs significantly more.

The interesting thing about looking at all this from an analytical perspective is that you can scale a bullet up exactly and the BC will increase because sectional density will increase. Weight increases with the cube of scale while frontal area is only a square function. As such a bullet of the exact same proportions 2x as large as another will have 2x the sectional density because it weighs 8x as much while the frontal area only increases by 4x. That explains why on average larger calibers can have higher BCs. The .30 cals are at a general handicap though because the form factors are all sub par, especially when compared to a good 6, 6.5, 7 or .338 bullet.

If you want to think about something interesting, consider a bullet specially made to be fired with a sabot from a larger caliber rifle. You could optimize the bullet shape and length to get a very high BC and provided a fast enough rate of twist or dense enough material (tungsten or DU come to mind) stability could be achieved. Adding the extra area at the base from the sabot overcomes the internal ballistics limitations on velocity of the projectile caliber. The result would be a bullet with a very high BC and very high velocity but less recoil than if the same were attempted with a "conventional" design, i.e. jacketed lead core, full-bore bullet. Using a .224 projectile in a .338 sabot for example, you would need a 125gn projectile in .224 to get the same SD as a .338 bullet of 285gn. Getting such a projectile to have a good form factor wouldn't be difficult and using a high density material would address the length/stability issue. You should then be able to fire such a bullet to either equal the 285gn bullet's trajectory with a >50% reduction in recoil or greatly exceed the 285gn performance with a smaller reduction in recoil. I know that there have been sabot rounds in the past, but they predominately used light bullets and were only looking for speed numbers. The exterior ballistics were quite poor on the whole.

murf
January 19, 2013, 02:59 PM
d2wing,

wind drift in one easy sentence! (well maybe not that easy) force over time is distance. thank you for that.

murf

d2wing
January 19, 2013, 05:26 PM
Helo that proves what works in theory doesn't work in practice because your theory is not complete. Scale matters. This is long proved in practice. Range matters because you have to take into consideration things like sonic transition, mass to surface and momentum and stability. All this has been long be figured out. That is why the military uses .338 and .50 cal for long range snipers. No one uses 2 ft long .22 cal. Bullets. Keep thinking though, someone always comes up with something new. Shooting is proof. Results mean more than any theory.
You do have a point as spent uranium has been used to add mass so it must work. But I don't think that is practical.

Big JJ
January 19, 2013, 06:38 PM
Are bullet BCs calculated by the mfgs with consideration to the length and surface area or are they calculated just using the frontal shape and the weight?
Has anyone considered that two bullets of identical grain weight and caliber can have the exact same shape but one will be longer than the other if one is made of copper and the other made of lead?
This gives the longer one (the copper) a greater surface area and allows it to stabilize in the air better.
However this bullet (the copper) will be affected more by outside forces than the shorter (the lead) bullet.
Do those two bullets have a different BCs from the mfgs or would they be the same.
If so this will affect all shooting solutions.
This only comes up because I shoot on both sides of the Cali Condor zone.

sixgunner455
January 19, 2013, 08:10 PM
d2wing - DU is practical, but only in the right gun. Shoulder fired rifles, I don't think so.

helotaxi
January 19, 2013, 11:35 PM
Helo that proves what works in theory doesn't work in practice because your theory is not complete. Scale matters. This is long proved in practice. Range matters because you have to take into consideration things like sonic transition, mass to surface and momentum and stability. All this has been long be figured out. That is why the military uses .338 and .50 cal for long range snipers. No one uses 2 ft long .22 cal. Bullets. Keep thinking though, someone always comes up with something new. Shooting is proof. Results mean more than any theory.
You do have a point as spent uranium has been used to add mass so it must work. But I don't think that is practical.The mil used/uses the .50 because it's a NATO standard cartridge and was the largest that they could realistically cram into a shoulder fired rifle. The ballistics are excellent as is downrange energy. They were more interested in the energy portion since the .50 is considered an anti-material weapon. They developed the .338LM because the .50 was too heavy, recoiled too much and had way more energy than needed for anti-personnel work even at over 1.5 miles.

As far as the transonic transition, mass doesn't play as much a role as bullet shape does. Mass has nothing to do with stability. Mass to surface is sectional density which has been more than covered. 90gn .224 bullets of standard construction are available right now. Using different material to make a 125gn bullet isn't that cosmic and a tungsten bullet could be shorter than a 75gn Amax being that tungsten is 1.7 times more dense than lead.

The idea is that you've got a bullet with a G7 BC better than .356 that you can comfortably fire in the 3600fps range from a .338LM rifle without a muzzle brake. The resulting exterior ballistics should be impressive to say the least. Like half the drop of a .338LM bullet at 2000yds. An extra 700yds before the bullet enters the transonic region. It would have plenty of energy for anti-personnel work out well past a mile and will have enough sectional density that it would punch right through body armor at that range. Probably engine blocks as well.

helotaxi
January 19, 2013, 11:51 PM
Has anyone considered that two bullets of identical grain weight and caliber can have the exact same shape but one will be longer than the other if one is made of copper and the other made of lead?
This gives the longer one (the copper) a greater surface area and allows it to stabilize in the air better.
However this bullet (the copper) will be affected more by outside forces than the shorter (the lead) bullet.
Do those two bullets have a different BCs from the mfgs or would they be the same.The longer bullet is not more stable in the air, it doesn't work that way. In fact, the opposite is pretty much true. Longer bullets require a higher rate of spin to stabilize and can be very temperamental with stability.

As far as the effect that making a bullet from solid copper has on BC...

There is a practical limit on bullet length (as a ratio to the bullet caliber) both because you can only reasonably go with so fast a rate of twist and you still have to get the bullet in the case with room for powder and in the magazine. The end result is that on the whole, monolithic copper bullets have low BC's because for a given length they have a low sectional density compared to a lead core bullet. The Barnes bullets on the whole have a pretty good form factor, but the sectional density is a serious limiting factor. Hornady and Nosler advertise the same BC for their monolithic bullets of a given weight and caliber as the Interbond and Accubond respectively, but I'm not sure if I believe it. I think that they're putting all the extra length in the bullet shank, leaving the ogive and boat tails the same as the lead core bullets. That should make for very close to the same BC, but exactly the same... The problem is that you run out of length before you get to the higher weights when using a monolithic bullet thus limiting the highest possible BC.

Big JJ
January 21, 2013, 10:03 AM
Helotaxi/Team
So two bullets shot out of the same gun with all other factors being the same except the lead vs copper bullet the lead will stabilize better than the copper???
When I load copper I use Barnes bullets and Barnes load data.
When loading lead I use load date that will give me the same FPS to match the Barnes FPS and the powder or bullet mfg formulas.
When I put the 200 grain lead next to the 200 grain copper bullet the copper is noticeably longer.
My gun is a Remington model 788,308win with a 1 to 10 twist.
The copper always shoot a better group off of a bench rest than my lead bullets.
I always thought it was due to the length of the bullets and there ability to stabilize in the air.
Given the facts you have stated above it makes me think that I should be looking for other ways to increase the accuracy of my lead bullets.
What do you guys think???

helotaxi
January 21, 2013, 10:53 AM
Helotaxi/Team
So two bullets shot out of the same gun with all other factors being the same except the lead vs copper bullet the lead will stabilize better than the copper???
When I load copper I use Barnes bullets and Barnes load data.
When loading lead I use load date that will give me the same FPS to match the Barnes FPS and the powder or bullet mfg formulas.
When I put the 200 grain lead next to the 200 grain copper bullet the copper is noticeably longer.
My gun is a Remington model 788,308win with a 1 to 10 twist.
The copper always shoot a better group off of a bench rest than my lead bullets.
I always thought it was due to the length of the bullets and there ability to stabilize in the air.
Given the facts you have stated above it makes me think that I should be looking for other ways to increase the accuracy of my lead bullets.
What do you guys think???
Stability and accuracy are not the same thing. Bullet consistency and matching the powder charge and bullet to the barrel harmonics as well as barrel quality determine accuracy for the mast part, but that assumes that the bullets is stable. For the bullet to be stable in flight, it must leave the muzzle without excessive yaw (mostly a barrel and bullet quality thing) and be spinning fast enough to be gyroscopically stable. The longer the bullet, the faster it must be spun to be stable. As long as the bullet is stable, that's all you really care about from a practical standpoint. There are some that argue "degrees of stability" but for all practical purposes, it's a binary question "is the bullet stable or not?"

Matching velocity between two completely different bullets is usually not the formula for getting one to shoot as well as the other. If you can get them stable, the Barnes bullets often can shoot very well simply because they are very consistent. You don't have issues with jacket thickness inconsistencies or the bullet imbalance that they can cause. They usually have a fairly long bearing surface and that means that they will usually enter the bore straight. All of these things are very conducive to getting very consistent results from shot to shot. On the other hand, some rifles have an inadequate rate of twist to get the longer monolithic bullets stable or leaves them right at the ragged edge. Unstable means that you'll likely never hit the target, while marginal stability usually ends with very inconsistent accuracy.

d2wing
January 21, 2013, 06:06 PM
A bullet of greater diameter will have more rotational enertia and well as longitudinal enertia. So just scaling down a bullet doesn't work. Actual bullet engineers know that their are optimal shapes, sizes and weights that consider all factors. As hello said, much depends on quantity of the bullet, seating in relation to lands and optimal load for each design bullet.
Making a bullet longer for the same mass makes it less stabile as a general rule if it is already at optimum length.
In the case of the copper bullet, it also could be that the more consistent weight distribution as lead bullet the cog is behind the midpoint of the bullet. Also the bullet could engage the lands sooner as it is longer. I dunno.

d2wing
January 22, 2013, 09:26 AM
I am sorry, what I said is about maximum ranges. Helo is right about scaled down ranges. There are optimum efficiencies at scaled down ranges. Bench rest shooter are striving for these goals and have down a wonderful job of refining the optimum caliber projectiles, speed twist rates, and many other factors such as quality of components, chamber and cartridge, primer, each variable carefully developed for the range to shoot at. It is not about maximums but optimums and quality control. Neither fat or skinny is a good rule of thumb. Refinement is.

Big JJ
January 22, 2013, 08:14 PM
d2wing and Helo
Thank you for clearing up some of my assumptions.
I am considering casting my own bullets in 308 and 357.
I would like to get the most consistant range rounds that I can get.
It looks like Lee mfg is the best bullet mold maker that I can find at resonable price. That is currently my choice. I am open to suggestions.
What do you guys consider to be the optimum bullet grain weight and shape for the these two calibers and what should the best FPS be for the best accuracy.
This seems to be a mathmatical problem that I have no idea how to solve.
There are obverously more issues to consider than than the 3 I have asked about above however it does seem to me that there should be a optimum bullet for the two calibers.
Maybe there is a data base for this type of info that I should be looking at???
Maybe just use the info from a bullet mfg and take the bullet with the highest BC for the caiber???
Thanks Big JJ

RPRNY
January 22, 2013, 08:24 PM
JJ - optimal bullets will depend on YOUR rifles. Ballpark optimal is somewhere around the longest that will consistently stabilize out to the range you desire and, if used for hunting, deliver optimal terminal ballistics, ie controlled, optimized expansion. This will in turn be determined by powder charge - type of powder and amount - as this will determine velocity and terminal ballistics at a given range.

I would suggest that before buying bulllet molds, you buy a selection of the most likely bullets cast from the molds you are looking at (Montana Bullet Works, Oregon Trail, Beartooth etc., to name but a few suppliers) and that you develop some loads (Lyman always has good cast data) to see what is working well for you in your rifles. THEN buy the molds.

You may also want to go http://castboolits.gunloads.com/forum.php and http://www.castbulletassoc.org/for example to do some more research.

d2wing
January 22, 2013, 10:58 PM
With cast bullets high BC isn't an issue because you can't push them real fast anyway as far as I know. I did cast 357 bullets years ago and used a luber sizer and gas checks.
As I recall I used a Elmer Keith hot .38 load and a Semi wadcutter bullet design.
Casting .308 is way over my pay grade. As Rprny said you need to see the guys that know.

Big JJ
January 24, 2013, 10:17 AM
Team
Thanks for the advice..
I am sure that it has saved me a lot of headaches.

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