B.C. IM CONFUSED ON THE 6.5 CALIBER

[Steelflight]

IM hoping one ofthe many experts can atleast pm me on this.

The more I read and study how BC works. The more I scratch my scratch head. According to a basic read. If all is equal. Its weight that finally decides. So I wonder. Why does the 6.5 165grain claim to handle wind drift at least as good or better then a 180 .308 in the same shape?

 

[The Oregonian]

You also have to take the caliber of the bullet into account...also think of BC as, in a sense, how streamlined the bullet is. All else equal (caliber, bullet shape), a heavier bullet will be a little longer and more streamlined.

A 6.5 is smaller caliber than a .308, so not all else is equal with those two bullets.

 

[Steelflight]

I guess what IM trying to solve is how the 6.5 scores higher BC than .308. Until you get in the 200 grain weight class.

I am comparing calibers and still the question marks abound

 

[Deleted member 103481]

BC = sectional density/form factor

Sectional density is weight/ (diameter x diameter)

Form factor is drag coefficient/drag coefficient of a standard bullet

Because the diameter of the .308 bullet is wider it is a larger number that the grains are being divided by.

In your scenario we have the same form factor, so let's just talk about sectional density given your bullet weights...

The 6.5 will have a higher BC in this scenario because the sectional density is much higher.

 

[tailbon3]

Bullet weight isn't the only factor. Given two bullets of the same weight and general shape, the smaller caliber bullet will slip through the wind better. So, a 150 grain 308 has to push more air out of the way than a 150 grain 6.5 mm bullet

 

[Steelflight]

DING! Light bulb just turned on. I get it.

Thank you all. I now feel llike should have caught that

 

[WeiserBucks]

That's what we're here for, good luck moving forward.

 

[ShtrRdy]

I had to read one of Brian Litz's books before I understood the relationship. jmcmath described it well.

Now I would like to better understand how the BC is used in a ballistic program.

 

[Susquatch]

Please allow me to add another ballistics insight for you.

As Isaac Newton discovered a very long time ago, all objects (bullets in our case) drop to the ground at virtually identical rates in response to gravity. They start falling at zero velocity and accelerate toward the ground more or less identically regardless of weight, size, or shape.

How much any bullet (all bullets) drops is strictly and unconditionally related to how long they have been dropping. Time and time alone defines how far a bullet drops. The more time, the more drop. The less time, the less drop.

A perfect bullet going twice as fast in the forward direction takes exactly the same time as the slower bullet to reach the ground. HOWEVER, because it's going twice as fast in the forward direction, it covers twice the distance in the time it takes to reach the ground. Or expressed a different way, it reaches the target twice as fast so it has only fallen half of the time and so it drops less than half the height (drop). I say "less than half" because it all gets a bit more complicated when you realize that all bullets ACCELLERATE faster and faster toward the ground, but DECELLERATE horizontally as they fly toward the target.

The ballistics coefficient, drag, and weight all define how quickly the forward velocity deteriorates. A slippery pointy bullet slows down more slowly than a flat head bullet. That means the slippery bullet covers more ground before it falls to the ground or drops a given height. This effect becomes more and more important the further out the distance is.

However, the time to fall to the ground stays virtually the same for all bullets regardless of all those parameters.

So, for any bullet over a given distance:
1. Faster means less drop because time interval to drop is shorter and therefore drop is less
2. More slippery for the same initial velocity means less drop because bullet slows down less and therefore covers more ground horizontally
3. More weight for the same velocity, shape, and size means less drop because of conservation of forward momentum

At its core, drop is easy to calculate if you know the time interval between here and there.

However, knowing the time interval is the hard part! That's really what ballistics is all about. That pesky ballistic coefficient in all of its varients, are all attempts to define mathematically how quickly a bullet decellerates in the horizontal direction due to the resistance of the air it travels through. If we know that, then we can calculate the time interval, and then the drop is easy peasy!

So that's the insight. Ballistics is really all about time.

 

[Steelflight]

Time spent at the range!

 

[Susquatch]

Just thinking a bit more about your question and thought the following might help too. It's a cut and paste of a comment I made on a different thread on a related subject.

"Some time ago I wondered why the average 6.5 twist was so fast compared to other calibers. (Picking just two examples smaller and larger, the avg 243 rifle is 10, avg 308 is 10, but the average 260 is 8). So I did some research. It turns out that the avg 6.5 bullet is quite a bit longer than other bullets for their caliber. If you make a statistical bar chart of all available bullets of all calibers by caliber you will end up with a fairly linear distribution for everything EXCEPT the 6.5/264. For some reason, they are different and have a scatter bar more like the 7mm but down two calibers - a significant "bump" or anomoly in the curve. This doesn't need to be so, but it is! I do not know for sure, but I think this is probably an artifact of the original long range competition intent of the 6.5s and also explains their relatively high ballistic coefficients, pencil bullets, and therefore higher twist rates. The bottom line is that the average 6.5/264 bullet is longer than other calibers for its caliber or better said, the average 6.5 bullet has a higher sectional density than the average bullet in other calibers."

As mentioned, this didn't have to be so. The average 6.5 could just as easily have had shorter lighter bullets and slower twists. And indeed some do. But it just isn't the average. For the 6.5 (and only the 6.5) the AVERAGE bullet is longer and heavier for its caliber than the other bullets are for their calibers.

But apparently it didn't go that way. The bullet and rifle makers all opted for longer heavier bullets and fast twists. It's almost like they all got together and said "Let's make the 6.5 a caliber (regardless of cartridge) that is primarily intended to be used for long range competition".

 

[cohunt]

To a poi

Please allow me to add another ballistics insight for you.

As Isaac Newton discovered a very long time ago, all objects (bullets in our case) drop to the ground at virtually identical rates in response to gravity. They start falling at zero velocity and accelerate toward the ground more or less identically regardless of weight, size, or shape.

How much any bullet (all bullets) drops is strictly and unconditionally related to how long they have been dropping. Time and time alone defines how far a bullet drops. The more time, the more drop. The less time, the less drop.

A perfect bullet going twice as fast in the forward direction takes exactly the same time as the slower bullet to reach the ground. HOWEVER, because it's going twice as fast in the forward direction, it covers twice the distance in the time it takes to reach the ground. Or expressed a different way, it reaches the target twice as fast so it has only fallen half of the time and so it drops less than half the height (drop). I say "less than half" because it all gets a bit more complicated when you realize that all bullets ACCELLERATE faster and faster toward the ground, but DECELLERATE horizontally as they fly toward the target.

The ballistics coefficient, drag, and weight all define how quickly the forward velocity deteriorates. A slippery pointy bullet slows down more slowly than a flat head bullet. That means the slippery bullet covers more ground before it falls to the ground or drops a given height. This effect becomes more and more important the further out the distance is.

However, the time to fall to the ground stays virtually the same for all bullets regardless of all those parameters.

So, for any bullet over a given distance:
1. Faster means less drop because time interval to drop is shorter and therefore drop is less
2. More slippery for the same initial velocity means less drop because bullet slows down less and therefore covers more ground horizontally
3. More weight for the same velocity, shape, and size means less drop because of conservation of forward momentum

At its core, drop is easy to calculate if you know the time interval between here and there.

However, knowing the time interval is the hard part! That's really what ballistics is all about. That pesky ballistic coefficient in all of its varients, are all attempts to define mathematically how quickly a bullet decellerates in the horizontal direction due to the resistance of the air it travels through. If we know that, then we can calculate the time interval, and then the drop is easy peasy!

So that's the insight. Ballistics is really all about time.

want more confusion?

To a point, technically air resistance also affects the max rate of fall, once the force of air resistance equals the gravitational force then the object has met its terminal velocity and no longer accelerates towards earth any faster no matter how long it is in flight.
Gravitational acceleration is 9.8m/s/s until it hits its terminal velocity, but not every object has the same terminal velocity
The shape of the object affects its coefficient of drag (Cd) which In turn affects its terminal velocity.
Think of a flat piece of paper weighing 1 gram, it will not drop as fast as a rock weighing 1 gram due to its air resistance
Though it is small in the big picture, an object with more surface area but the same weight will stop accelerating due to gravity sooner than one with a smaller surface area

 

[Susquatch]

...... To a point, technically air resistance also affects the max rate of fall, once the force of air resistance equals the gravitational force then the object has met its terminal velocity and no longer accelerates towards earth any faster no matter how.........

Yup, hence my choice of the word "virtually". For our case, with lead bullets and copper jackets, (no sheet of paper or feathers), the relatively short time and distance of the drop, and the very low drop velocity, the difference is insignificant and can be ignored for the purpose of the discussion.

However you are totally correct about this. I knew that going in but didn't think anyone would challenge it so I deliberately deleted a paragraph I had originally included to describe it because it was too confusing and didn't really matter in the big picture. Therefore I used the word "virtually" and other similar terms instead when discussing this minor effect.

I also avoided discussing temperature, pressure, and humidity even though these have an even bigger affect on the results. I really only wanted to help other readers understand that it's all about time of flight because that insight really helps the shooter understand what is happening.

 

[Silly_Ghillie]

nevermind, I didn't see the part where you got it.