How are BCs Used to Calculate Drops & drifts?

[Plumbum82]

Something has me greatly confused. Imagine a .25 bullet, now a .25 bullet 4x the length and a .50 of the same length just fatter, so longy and fatty weight the same. My common sense tells me, the longy will weigh 4x but have 4x the side-on surface area, and so should blow just as much, but retain velocity 4x as efficiently because the front has the same SA. As for the .50 it also will weigh 4x but have 4x frontal SA, so decelerate just as fast, but on it's side it will only have 2x the SA over 4x mass and so should defeat wind twice as well.

With all that being said, how on earth can 1 number describe wind resistance properties when blowing on 2 different sides of an unsymmetrical shape, I can only see that working when talking about spheres.

 

[Fast14riot]

Following to see what some others have to say.

 

[BoydAllen]

Something has me greatly confused. Imagine a .25 bullet, now a .25 bullet 4x the length and a .50 of the same length just fatter, so longy and fatty weight the same. My common sense tells me, the longy will weigh 4x but have 4x the side-on surface area, and so should blow just as much, but retain velocity 4x as efficiently because the front has the same SA. As for the .50 it also will weigh 4x but have 4x frontal SA, so decelerate just as fast, but on it's side it will only have 2x the SA over 4x mass and so should defeat wind twice as well.

With all that being said, how on earth can 1 number describe wind resistance properties when blowing on 2 different sides of an unsymmetrical shape, I can only see that working when talking about spheres.

As part of the design process BCs are calculated and then confirmed by velocity and/or time of flight measurements Higher BCs show less velocity loss at a given distance, and shorter time of flight.

Books have been written on the subject of ballistics, and there is a lot of information available on line. I suggest that if you really want to know more, that you locate these resources and spend a lot of time studying. Having an excellent math background really helps.

You may find this information ,from the Berger website, helpful.

Measuring BCs

Measuring Ballistic Coefficients (BCs) is one of those things that’s easy to do in principle, but very difficult to do accurately in practice. Remember, BC is most directly described as the ability of the bullet to retain velocity. So, the most direct way to measure BC is to measure the

bergerbullets.com

 

[Plumbum82]

As part of the design process BCs are calculated and then confirmed by velocity and/or time of flight measurements Higher BCs show less velocity loss at a given distance, and shorter time of flight.

Books have been written on the subject of ballistics, and there is a lot of information available on line. I suggest that if you really want to know more, that you locate these resources and spend a lot of time studying. Having an excellent math background really helps.

You may find this information ,from the Berger website, helpful.

Measuring BCs

Measuring Ballistic Coefficients (BCs) is one of those things that’s easy to do in principle, but very difficult to do accurately in practice. Remember, BC is most directly described as the ability of the bullet to retain velocity. So, the most direct way to measure BC is to measure the

bergerbullets.com

I see, because there seems to be a lot of people who think that BC can predict wind drifts as well, I also thought that was the case until I recently thought about it and realised it doesn't make sense.

 

[jelenko]

I see, because there seems to be a lot of people who think that BC can predict wind drifts as well, I also thought that was the case until I recently thought about it and realised it doesn't make sense.

No. BC does predict bullet drift.
In your scenario, you need to include the aerodynamic effects due to the form/shape of the bullet.

It's the same with planes. High speed, > Mach 1 planes all want a pointy nose and a teardrop body.

@HappyHellfire would be able to explain more thoroughly.

 

[chop house]

Perhaps the way to envision this is that bc is a derivative of time of flight, and time of flight influences wind drift. In the articles there also is a little gem about the bullet turning into the wind (airflow) when fully stabilized

 

[Alex Wheeler]

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.

 

[STORM2]

Home - Applied Ballistics

appliedballisticsllc.com

Buy the books and/or enroll for $10/month to learn more than most of us need. You will save enough in ammo to pay what you spend.

 

[ebb]

Gravity is constant, wind is not

 

[Plumbum82]

No. BC does predict bullet drift.
In your scenario, you need to include the aerodynamic effects due to the form/shape of the bullet.

It's the same with planes. High speed, > Mach 1 planes all want a pointy nose and a teardrop body.

@HappyHellfire would be able to explain more thoroughly.

I'd be assuming they're literally just cylinders for the sake of fairness, only considering length, width and mass.

 

[davidjoe]

Something has me greatly confused. Imagine a .25 bullet, now a .25 bullet 4x the length and a .50 of the same length just fatter, so longy and fatty weight the same. My common sense tells me, the longy will weigh 4x but have 4x the side-on surface area, and so should blow just as much, but retain velocity 4x as efficiently because the front has the same SA. As for the .50 it also will weigh 4x but have 4x frontal SA, so decelerate just as fast, but on it's side it will only have 2x the SA over 4x mass and so should defeat wind twice as well.

With all that being said, how on earth can 1 number describe wind resistance properties when blowing on 2 different sides of an unsymmetrical shape, I can only see that working when talking about spheres.

The way it works is that BC is always changing across the bullet’s velocity.

If the bullet isn’t moving at all relative to a side wind, then the profile that the side presents is absolutely relevant to how it moves, the same way as if it was propelled sideways into no wind at all.

The bullet in flight will see “mainly” a headwind but like a weathervane or flag it will point in the direction where pressure is equal on both sides.

Like a weathervane, the larger profile rear/side has more to do with wind resistance than the much smaller front does, and forces the front to point directly into the total wind (wind vector), even though the bullet may now not be pointed precisely at the target. Front and rear in this case is defined by what is behind the center of gravity.

A strip of paper rolled into a cylinder around a sphere, that precisely contains the sphere, has exactly the same surface area as the sphere.

 

[davidjoe]

The weathervane, flag or stable aircraft with tail planes doesn’t 100% capture a rear-heavy bullet’s flight like it does an arrow or dart’s, with all its weight in the front, such that air passing over the feathers force it to fly point first. I think this occurs, but at the same time I don’t believe a cylinder necessarily requires an asymmetrical point and tail, or a varying center of pressure for stability. A football is not asymmetrical and can be thrown with perfect stability. A nearly pure cylinder is basically was hard cast large caliber revolver bullets actually are.

 

[SteveOak]

@HappyHellfire would be able to explain more thoroughly.

LOL

 

[Plumbum82]

The weathervane, flag or stable aircraft with tail planes doesn’t 100% capture a rear-heavy bullet’s flight like it does an arrow or dart’s, with all its weight in the front, such that air passing over the feathers force it to fly point first. I think this occurs, but at the same time I don’t believe a cylinder necessarily requires an asymmetrical point and tail, or a varying center of pressure for stability. A football is not asymmetrical and can be thrown with perfect stability. A nearly pure cylinder is basically was hard cast large caliber revolver bullets actually are.

Yeah, most bullets rely purely on spin for stability, pointed rifle bullets actually want to fly backwards on their own, because on their side the point has greater surface area for it's mass than the shaft and so defects wind worse than the rear.

 

[davidjoe]

Yeah, most bullets rely purely on spin for stability, pointed rifle bullets actually want to fly backwards on their own, because on their side the point has greater surface area for it's mass than the shaft and so defects wind worse than the rear.

Yes, and I have read that when loaded backwards, they do work surprisingly well.

 

[BoydAllen]

I see, because there seems to be a lot of people who think that BC can predict wind drifts as well, I also thought that was the case until I recently thought about it and realised it doesn't make sense.

Wind drift relates to muzzle velocity and B C. Comparing two different bullets at the same muzzle velocity, the bullet with the higher BC will have less wind drift.

 

[ebb]

Arrows always wanted to have more front of center weight to fly well.

 

[Richard Jones]

A strip of paper rolled into a cylinder around a sphere, that precisely contains the sphere, has exactly the same surface area as the sphere.

Not following this. With each layer of paper, wouldn't the SA increase? The inside of the strip would have the same SA as the sphere, but the outside SA would be more. Conundrum theories abound...

 

[wwbrown]

The wind drift equals the wind speed times the difference in time of flight the bullet takes to reach the target range minus the time of flight that same bullet would have taken if it was in a vacuum. The actual time of flight is a function of the muzzle velocity, environmentals (including wind) and the ballistic coefficient, this is where the BC raises its ugly head in wind drift.

I think the mis-match between actual and our predicted wind drift is due to the variability of wind, the wind is not in a constant direction nor speed along the bullet's trajectory/ We make our drift estimates based upon what is probably an aggregate average of what we think the wind values are. In other words I believe the errors in our estimate of the wind are far greater than the errors in all of the other factors impacting the bullet's trajectory.

 

[davidjoe]

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.

I’m not aware of the constituent factors upon which BC is calculated, taking into account rifling deformation.