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

Discussion in 'Rifle Country' started by Jasper1573, Jan 17, 2013.

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  1. Jasper1573

    Jasper1573 Member

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    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).
     
    Last edited: Jan 17, 2013
  2. MErl

    MErl Member

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    is this true when you get up to the really big things like 30mm as well?
     
  3. cal30_sniper

    cal30_sniper Member

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    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.
     
  4. Jasper1573

    Jasper1573 Member

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    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.
     
  5. Jasper1573

    Jasper1573 Member

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    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
     
  6. cal30_sniper

    cal30_sniper Member

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    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.
     
  7. Walkalong

    Walkalong Moderator

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    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.
     
  8. Jasper1573

    Jasper1573 Member

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    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.
     
  9. Walkalong

    Walkalong Moderator

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    For a lighter and heavier bullet starting at the same speed both with the same BC it would be true.
     
  10. cal30_sniper

    cal30_sniper Member

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    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.
     
    Last edited: Jan 17, 2013
  11. cal30_sniper

    cal30_sniper Member

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    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".
     
  12. jmr40

    jmr40 Member

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    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.
     
  13. Jasper1573

    Jasper1573 Member

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    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.
     
  14. cal30_sniper

    cal30_sniper Member

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    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.
     
  15. RPRNY

    RPRNY Member

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    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 ;-)
     
  16. cal30_sniper

    cal30_sniper Member

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    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.
     
  17. Jasper1573

    Jasper1573 Member

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    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;-)
     
  18. cal30_sniper

    cal30_sniper Member

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    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...
     
  19. Jasper1573

    Jasper1573 Member

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    That's a good one!

    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!
     
  20. Abel

    Abel Member

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    uhh....

    ...30-06!

    yeah.
     
  21. conrad427

    conrad427 Member

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    My thoughts exactly, Abel!
     
  22. TNBilly

    TNBilly Member

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    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.
     
  23. helotaxi

    helotaxi Member

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    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.
     
  24. helotaxi

    helotaxi Member

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    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.
    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.
     
  25. cal30_sniper

    cal30_sniper Member

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    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.
     
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