How are BCs Used to Calculate Drops & drifts?

[divingin]

I have suspected that we do realize lower wind drift performance from lighter bullets with the same BC as heavier bullets, like Alex mentions, and that the reason for this is that the deformation of rifling grooves on the body of bullets, 1) promotes lateral drag in crosswind relative to a perfectly smooth body, and 2) the ratio of deformation caused by rifling to the bullet’s weight, is higher in lighter bullets compared to heavier bullets having the same BC.

Isn't that mostly a function of the wind energy acting on a lighter vs heavier object? A 10 mph constant wind will blow a ping pong ball a lot further than a similar sized ball bearing.

 

[davidjoe]

Isn't that mostly a function of the wind energy acting on a lighter vs heavier object? A 10 mph constant wind will blow a ping pong ball a lot further than a similar sized ball bearing.

It’s very possible to have a long .223 and medium .308 have exactly the same BC.

Most guys don’t realize this, but you do NOT need to type in caliber or bullet weight to get a trajectory and wind drift calculation. You can leave it blank, go with the default, or type in the actual, but the results are the same.

BC and velocity are the only factors that actually affect drop and drift.

 

[wwbrown]

It’s very possible to have a long .223 and medium .308 have exactly the same BC.

Most guys don’t realize this, but you do NOT need to type in caliber or bullet weight to get a trajectory and wind drift calculation. You can leave it blank, go with the default, or type in the actual, but the results are the same.

BC and velocity are the only factors that actually affect drop and drift.

The only factors you have some control over, the environmentals being the uncontrollable part timing can give you some control.

 

[Plumbum82]

Nobody has mentioned caliber. Alex mentioned heavier bullets, but i'm more so talking about Calibre and how that effect SA. Larger shapes are are more efficient cramming mass than smaller ones, 2x diameter means 2x SA on it's side but 4x mass, so 2x momentum when factoring in drag over SA. Where as a 4x long, 4x SA and 4x heavy bullet should deflect the same over the same time, only defecting less down range as a result of retaining speed better and having lower travel times. Am I wrong?

 

[JLT]

It’s very possible to have a long .223 and medium .308 have exactly the same BC.

Most guys don’t realize this, but you do NOT need to type in caliber or bullet weight to get a trajectory and wind drift calculation. You can leave it blank, go with the default, or type in the actual, but the results are the same.

BC and velocity are the only factors that actually affect drop and drift.

David, that is how the calculator math works, however, a bullet's moment of inertia is relevant in actual wind drift. Mass has a part to play in Newtonian physics. Newton's First Law of Motion states that a greater mass requires greater force to affect its motion, axial or lateral. Newton's Second Law of Rotation puts a finer point on the effects of a mass with angular velocity. The calculators will produce the same number, but actual drift will be different if BCs are the same and the masses are different. Given identical BCs, a bullet of greater mass will drift less.

 

[CharlieNC]

Ballistic calculations are based on either an ideal G1 or G7 standard. Calculations for the bullets we shoot use a nominal BC referenced to the standard to obtain an approximation, which we then tune via velocity and BC adjustments to further improve the approximation. Or pay to buy a custom drag curve!

 

[davidjoe]

David, that is how the calculator math works, however, a bullet's moment of inertia is relevant in actual wind drift. Mass has a part to play in Newtonian physics. Newton's First Law of Motion states that a greater mass requires greater force to affect its motion, axial or lateral. Newton's Second Law of Rotation puts a finer point on the effects of a mass with angular velocity. The calculators will produce the same number, but actual drift will be different if BCs are the same and the masses are different. Given identical BCs, a bullet of greater mass will drift less.

On mass, I have thought and taken for granted they had already baked in, if you will, that effect in cross sectional density or some other component of BC, such that it is complete enough in what it encompasses to call it an actual coefficient that we could directly compare different bullets drift without knowing more than their BC. But what you are saying is consistent with what we all observe in shooting heavier and lighter bullets, an apparent divergence.

On moment of inertia and drift, I have wondered if lateral momentum (greater spin) makes it “harder” to either upset, or push a faster spinning bullet off its path compared to a marginally stable bullet, as it is with a spinning top or gyroscope. The faster it spins the more it resists being axially titled.

We don’t read much about any benefits of over stabilization, but that might actually be one; usually we read only of detractions, such as the bullet “tricycling” to the target, lower realized BC, pressure at the case’s head, and heat damage.

We know that spin rate is not a component of published BC, as a manufacturer cannot know what twist a user will employ. I don’t think calculators request spin rate either. It might be applicable though, and even if there is no wind, slightly affect the length and height of spin drift.

It could very well be that drift is not affected by spin rate, and that’s why it’s left out, not because it’s unknown in advance. (A car’s front wheel assembly seems to work regardless of how fast the wheel is spinning after all.)

 

[JLT]

On mass, I have thought and taken for granted they had already baked in, if you will, that effect in cross sectional density or some other component of BC, such that it is complete enough in what it encompasses to call it an actual coefficient that we could directly compare different bullets drift without knowing more than their BC. But what you are saying is consistent with what we all observe in shooting heavier and lighter bullets, an apparent divergence.

On moment of inertia and drift, I have wondered if lateral momentum (greater spin) makes it “harder” to either upset, or push a faster spinning bullet off its path compared to a marginally stable bullet, as it is with a spinning top or gyroscope. The faster it spins the more it resists being axially titled.

We don’t read much about any benefits of over stabilization, but that might actually be one; usually we read only of detractions, such as the bullet “tricycling” to the target, lower realized BC, pressure at the case’s head, and heat damage.

We know that spin rate is not a component of published BC, as a manufacturer cannot know what twist a user will employ. I don’t think calculators request spin rate either. It might be applicable though, and even if there is no wind, slightly affect the length and height of spin drift.

It could very well be that drift is not affected by spin rate, and that’s why it’s left out, not because it’s unknown in advance.

As CharlieNC mentioned/implied, BC calculations are principally, if not exclusively, focused on trajectory. Precise wind drift predictions must take into consideration, among other things, the axial cross-sectional area which is largely independent of everything else that goes into the BC calculations.

Consider the 'claimed vs. real BC' comments we regularly encounter. Bullet manufacturers may not be using the same calculation and estimation methods, which leads to disagreement with published numbers (claimed) and on-target observations (real). Empirical methods are used more commonly these days to firm up the numbers for individual bullets - downrange Doppler radar velocity measurements have done much to improve the BC numbers from a growing number of manufacturers. Empirical methods may soon replace the G1/G7-plus-form factor models that preceded them. That tech is getting less expensive very rapidly.

For now, I'm sticking with the intuitively comfortable thought that, for a given BC, a heavier bullet will drift less.

 

[Plumbum82]

As CharlieNC mentioned/implied, BC calculations are principally, if not exclusively, focused on trajectory. Precise wind drift predictions must take into consideration, among other things, the axial cross-sectional area which is largely independent of everything else that goes into the BC calculations.

Consider the 'claimed vs. real BC' comments we regularly encounter. Bullet manufacturers may not be using the same calculation and estimation methods, which leads to disagreement with published numbers (claimed) and on-target observations (real). Empirical methods are used more commonly these days to firm up the numbers for individual bullets - downrange Doppler radar velocity measurements have done much to improve the BC numbers from a growing number of manufacturers. Empirical methods may soon replace the G1/G7-plus-form factor models that preceded them. That tech is getting less expensive very rapidly.

For now, I'm sticking with the intuitively comfortable though that, for a given BC, a heavier bullet will drift less.

My thought process says if you have a .308 bullet that tapers down to a .05 meplat, when looking at it on it's side the .308 shaft will have significantly higher cross-sectional density than the thin tip, so it would stand to reason that a round or flat nose bullet that's .308 right up to it's end would deflect less given the same travel time. It would also decelerate faster though, thus increasing travel times, so at a longer range may defect more, just a matter of how far until the improved velocity retention takes over. I started this forum because as i said, how can 1 number tell you deflection properties from 2 different sides of a non spherical shape, it'd be like saying a disk would fly just as well when thrown flat side forward as it does flat side down. That just sounds comical.

 

[JLT]

My thought process says if you have a .308 bullet that tapers down to a .05 meplat, when looking at it on it's side the .308 shaft will have significantly higher cross-sectional density than the thin tip, so it would stand to reason that a round or flat nose bullet that's .308 right up to it's end would deflect less given the same travel time. It would also decelerate faster though, thus increasing travel times, so at a longer range may defect more, just a matter of how far until the improved velocity retention takes over. I started this forum because as i said, how can 1 number tell you deflection properties from 2 different sides of a non spherical shape, it'd be like saying a disk would fly just as well when thrown flat side forward as it does flat side down. That just sounds comical.

What you're reading about is precisely why one number doesn't tell the whole story. Most BC calculators are, for the most part, concerned with time of flight as it effects trajectory. The force of gravity is a well-understood constant. Not very many pay heed to the effects of projectile mass and its effects on drift. Unless you really dig calculus and differential equations, it comes down to this:

Up and down is math. Side to side is voodoo.

On a relative basis, the lateral atmospheric effects are not well-predicted in most ballistic calculators. I have learned this theoretically and empirically. I have the three o'clock and nine o'clock nines to prove it.

 

[Plumbum82]

What you're reading about is precisely why one number doesn't tell the whole story. Most BC calculators are, for the most part, concerned with time of flight as it effects trajectory. The force of gravity is a well-understood constant. Not very many pay heed to the effects of projectile mass and its effects on drift. Unless you really dig calculus and differential equations, it comes down to this:

Up and down is math. Side to side is voodoo.

On a relative basis, the lateral atmospheric effects are not well-predicted in most ballistic calculators. I have learned this theoretically and empirically. I have the three o'clock and nine o'clock nines to prove it.

It's not what I've read, it's what I thought using my own deduction, I just find it hard to believe just how many supposed experts don't know about basic physics. I kind of expected to have people explain why I'm wrong but nobody has. I got E for physics and D in math, and can still tell you that bigger shapes hold more mass and volume compared to their surface area than small ones. 2x scale = 4x SA over 8x mass.

 

[Krogen]

Plug some numbers into an external ballistics program and you'll see that for a given ballistic coefficient, drift is independent of bullet mass. Try wild swings in bullet mass and there's no effect. This is because ballistic coefficient encompasses bullet mass. It's all about time of flight and the length of time a crosswind affects the bullet. A lower BC bullet takes longer to get to the target and slows more significantly than a higher BC bullet. Therefore a crosswind has a longer time of influence and subsequent wind drift on a lower BC bullet.

 

[Plumbum82]

Plug some numbers into an external ballistics program and you'll see that for a given ballistic coefficient, drift is independent of bullet mass. Try wild swings in bullet mass and there's no effect. This is because ballistic coefficient encompasses bullet mass. It's all about time of flight and the length of time a crosswind affects the bullet. A lower BC bullet takes longer to get to the target and slows more significantly than a higher BC bullet. Therefore a crosswind has a longer time of influence and subsequent wind drift on a lower BC bullet.

The problem with that is we'd be assuming wind effects all bullets equally, and therefore you'd only need consider travel times. But travel times are only half the equation, wind does not effect bullets the same on the side as the front, otherwise a tumbling bullet flying sideways would fly just as well as one pointed forward, which is absurd. A bullet's ability to deflect wind in front of it could only match the side if it is completely round. I'd need some real math to suggest otherwise.

 

[JEFFPPC]

Like you I dont beleive it does. BC does a great job of predicting drop. You can use that time of flight to give you some data to predict windage but in my experiance its no where near as accurate as the drop prediction. Because we shoot so many groups on paper with so many different bullets in the same condition we have seen that difference. Some bullets out perform or under perform that BC number for windage by a lot. Enough so I dont even look at BC as part of the process. If you want to know how well a bullet handles wind you have to shoot it next to others in the same relay and compare the trends over time. After seeing this time and time again thats my conclusion. One area that seems a little skewed is mass. In spite of it being part of the calculation, the heavier bullets always seem to do better that the lighter ones with similar BC.

A short answer with Alexs thoughts are would be......Trust you targets.

 

[wwbrown]

I believe the largest source of uncertainty when it comes to estimating wind drift is in the wind. I believe the wind id the largest source of error in any of the estimates.

I worked a program while at Lockheed that developed a device that measured the wind profile between the target and the shooter so it would spit out plots displaying the crosswind component of the wind. I never saw a plot of the wind that was anywhere what I would call steady wind.

How many times have you looked down the range and one flag was blowing to the right, the next to the left and the last hanging dead? You can't assign an aggregate average value to the wind in that situation. Maybe you think the BC is off by 10% your wind call (assuming you have something more than light winds) will probably be much further off than 10%.

When at your home range, if you have enough experience there, you can oftentimes make accurate wind calls based upon experience opposed to an estimate of the wind strength and going to a table.

 

[gpoldblue]

You can take that B.C. and $5 and go get you a cup of coffee and you'll come out way ahead.
B.C.'s are only valid in the conditions they were established in....and that was NOT in the real world.
B.C. changes with the direction you're shooting...the altitude....whether uphill/downhill.....air density....
wind direction/intensity...and other factors. B.C. also changes with velocity. Only testing on targets in your conditions will validate.
Good luck and stay safe.

 

[DocUSMCRetired]

I worked a program while at Lockheed that developed a device that measured the wind profile between the target and the shooter so it would spit out plots displaying the crosswind component of the wind. I never saw a plot of the wind that was anywhere what I would call steady wind.

Who did you work with? If you want to PM me I understand. The DARPA One Shot program was our baby. Two people currently working here now came from Lockheed, one of which was a lead guy on the One Shot program and also worked on laser defense systems.

By the way, the program still exists. But the LRF we make now that is capable of it is the size of a RAPTAR-S.

 

[DocUSMCRetired]

B.C. changes with the direction you're shooting...

Not exactly....

 

[gpoldblue]

Not exactly....

Try shooting in the same direction as the earth's rotation and then shooting the opposite direction of the earth's rotation and see the difference in point of impact on say a 1000yd target in each direction......the points of impact are not the same.

 

[davidjoe]

Not exactly....

I may be old school in my manner of thinking about this, but BC is just a relationship to the flight traits of certain arbitrarily ascertained projectiles. There are several prevailing forms of bullets and the similarity of the small arms projectile’s performance in question will more closely resemble one of them than the others, based principally on which one it comes closest to being a smaller version of.

As such, it is simplistically similar to saying this particular object is “.88 the mass of the standard kilogram” and that being the case I believe that the BC number assigned to a bullet need not consider any external variables at all.

Certainly the standard kilogram cylinder, of which there are a couple dozen distributed around the world (they are the same as humanly possible) will weigh less at different altitudes and over different densities of the earth’s sphere, as gravity is not equal everywhere, but none of the external factors are relevant in comparing an object’s mass to the mass of the standard kilogram, as mass is the physical characteristic of the set of number of molecules of a given type(s), not what a scale says.