Ballistic Coefficient

[Matt L.]

I'm confused about what the BC means. Does it have to do with trajectory? And do you want a higher number or a lower one? Thanks for all info.

[Vindalbakken]

The BC has to do with how easily the bullet will slip through the air. The lower BC bullets will shed speed more quickly because of the increased air resistance. This slowing down of course affects trajectory and downrange energy.

That is the simple answer.

[sdeviation]

and i also think that any bullet diameter with the same BC will fly the same given same speed threw the air...
to 300 yards i dont think it matters much .. but when u get way out younder a high BC is like a energy efficient rating...

[LongDraw]

If two bullets are of the same caliber and weight the one with the higher BC will have less drop, wind drift, and more retained energy. this becomes more recognizable the farther the bullet travels from the muzzle. It is a funky calculation of bullet length, mass, cross sectional area, density, and drag coefficient. = ballistic coefficient

All of this does not make a hill of beans difference for big game hunting bullets when shots are out to 300 yards or less.

Makes a huge difference for the Long Range guys that shoot out to 1000 yards.

[222rem]

If you want to know more about BC give the site below a read.


http://en.wikipedia.org/wiki/Ballistic_coefficient

[pogo]

BC (ballistic coefficient) is a reference to bullet/projectile efficiency. A measure of it's efficiency, if you will. The higher the number, the more efficiently/easily it passes through the air.

An airplane wing, for example, can generate a lot of 'lift', and it is said that it has a high 'coefficient of lift', but to do so it's shape must be thick with a lot of camber to the upper surface which also creates more drag as a by-product of generating the lift. i.e. it passes through the air less easily than a thinner wing.

With bullets(projectiles) there is no 'lift' to the equation, but there other variables that contribute to it's efficiency, such as shape(form drag), mass vs. length, even smoothness(laminar air flow) etc.

Roughly speaking, when you put your hand out your car window at 100 km/hr, palm down, that's high BC. If you turn your palm forward at 100 km/hr, that's low BC!

[Grizzly Adams]

The part that confuses me is how they derive that snake oil input that quantifies the bullet's profile., what Hatcher calls the form factor.
Grizz

[Matt L.]

Thanks for the info so far. If a bullet has a bc of .652 and another one has a bc of .435; which has the better bc? I'm slowly understanding but I've got a lot to learn yet lol!

[gube]

Originally Posted by Matt L.

Thanks for the info so far. If a bullet has a bc of .652 and another one has a bc of .435; which has the better bc? I'm slowly understanding but I've got a lot to learn yet lol!

The better B.C. would be .652

[johnhm]

www.chuckhawks.com/bc.htm

lots of examples here

[pogo]

Originally Posted by Grizzly Adams

The part that confuses me is how they derive that snake oil input that quantifies the bullet's profile., what Hatcher calls the form factor.
Grizz

Using the word "factor" sounds a bit confusing alright. i.e. what IS a 'factor'?

It can also be thought of as form "drag", a force acting against the bullet's energy. The form(shape)of a bullet directly affects it's efficiency. i.e. a ball and a bullet might have the same sectional density, but the shape(form) of the bullet makes it more slippery through the air and thus has a greater BC. This diagram might help:

http://www.centennialofflight.gov/es...drag/TH4G2.htm

Hatcher is very correct when he states that the form "factor" changes with velocity i.e. transonic, sub and super sonic. Airplane designers had all sorts of problems with this prior to supersonic flight. A wing would build up a pressure wave of compressed air ahead of it as speed increased. When the speed of the wing reached transonic values and began to overtake the pressure wave, the air flowing over the wing would become turbulent and adversely affect the control surfaces on the trailing edge. Eventually they realized that by designing a swept back wing, so that the wing did not go trans/super sonic all at once, it dramatically improved control and stability until all of the wing was going super sonic. Then they looked more at things like "form", or shape, to extend the distance the air would pass smoothly(laminar airflow) over the wing, thereby making it more "slippery"(less drag) and improving it's BALLISTIC COEFFICIENT, if you will.

For some calibres, using copper as opposed to lead will increase the BC because for the same mass/weight more can be done to optimize the form/shape of the bullet. If you look at manufacturer's charts, you can see that, generally, the high BC bullets carry more "thump" downrange.

I hope my analogous references to airplane wings helps.

Cheers,

[Grizzly Adams]

Originally Posted by pogo

Using the word "factor" sounds a bit confusing alright. i.e. what IS a 'factor'?

It can also be thought of as form "drag", a force acting against the bullet's energy. The form(shape)of a bullet directly affects it's efficiency. i.e. a ball and a bullet might have the same sectional density, but the shape(form) of the bullet makes it more slippery through the air and thus has a greater BC. This diagram might help:

http://www.centennialofflight.gov/es...drag/TH4G2.htm

Hatcher is very correct when he states that the form "factor" changes with velocity i.e. transonic, sub and super sonic. Airplane designers had all sorts of problems with this prior to supersonic flight. A wing would build up a pressure wave of compressed air ahead of it as speed increased. When the speed of the wing reached transonic values and began to overtake the pressure wave, the air flowing over the wing would become turbulent and adversely affect the control surfaces on the trailing edge. Eventually they realized that by designing a swept back wing, so that the wing did not go trans/super sonic all at once, it dramatically improved control and stability until all of the wing was going super sonic. Then they looked more at things like "form", or shape, to extend the distance the air would pass smoothly(laminar airflow) over the wing, thereby making it more "slippery"(less drag) and improving it's BALLISTIC COEFFICIENT, if you will.

For some calibres, using copper as opposed to lead will increase the BC because for the same mass/weight more can be done to optimize the form/shape of the bullet. If you look at manufacturer's charts, you can see that, generally, the high BC bullets carry more "thump" downrange.

I hope my analogous references to airplane wings helps.

Cheers,

Thanks Pogo, but how do you put a numerical value on all of the above? The formulas for calculating the BC, just gloss over that. I have two Hornady bullets here. Both are .277 dia weigh 150 grains and have very similar shapes, but one has a BC of .525 and the other is .462 The only difference is that form factor.
Grizz

[222rem]

Originally Posted by Grizzly Adams

Thanks Pogo, but how do you put a numerical value on all of the above? The formulas for calculating the BC, just gloss over that. I have two Hornady bullets here. Both are .277 dia weigh 150 grains and have very similar shapes, but one has a BC of .525 and the other is .462 The only difference is that form factor.
Grizz

Lets look at "very similar shape" and Ballistic Coefficient (BC)

270win 150gr bullet @ (we'll use easy number) 3000ft/sec shooting from rifle with 1:10 twist rate.

That means that bullet is traveling 3000ft/sec and spinning once every 10" of travel.When we do the math 12" X speed ÷ twist rate X 60 you'll notice that the bullet is turning 216000 revolutions per minute.(that's what keeps the bullets nose end forward) . That said you don't need much of change in shape to make difference in a drag of said bullet traveling through the air.
Also small difference not just in point of the bullet but base of the bullet makes a difference.
Just looking at the bullet you can't tell much, but send that bullet down the barrel @ 3000ft/sec , that is when even small difference in bullet shape makes the difference in BC.

If you hunt , lets say inside 300yds. don't lose any sleep over BC.
If you shoot 500 to 1000 yards then you might want the bullet with higher BC.

[Grizzly Adams]

Maybe, like the confused philosopher I am, I'm not making myself clear. I understand that the higher the BC, the more efficient the trajectory of the bullet, but in calculating the BC, the form factor is expressed as one definite numerical value. I'd like to know how the bullet makers arrive at that value, or do they just make these things up, on the fly.
Grizz

[pogo]

Originally Posted by Grizzly Adams

Maybe, like the confused philosopher I am, I'm not making myself clear. I understand that the higher the BC, the more efficient the trajectory of the bullet, but in calculating the BC, the form factor is expressed as one definite numerical value. I'd like to know how the bullet makers arrive at that value, or do they just make these things up, on the fly.
Grizz

Hmmmmm...

...first off, I'm no expert. I just find ballistics interesting. Like aerodynamics are interesting. Except with projectiles you just get one BIG, SUDDEN shot of horsepower at the beginning and then you are pretty much "coasting" the rest of the way!

That being said, this is the way I understand it works:

The term "form factor" is an expression for a, like you say, "snake oil" numeric value that is derived from an equation that uses a list of variables as long as your barrel. Aside from the variables that change from minute to minute when measuring bullet decelerations; things such as air density(pressure altitude and temperature), humidity, and skin friction, there are also the errors inherent in the measuring equipment, used in calculating, as accurately as possible the final ballistic coefficient, that contribute to the messy calculations! And none of them are 100% exact!

Form factor is a comparison of a particular bullet's coefficient of drag vs. the coefficient of drag of the standard model used. So when you ask, "what/how/why" about the "number", the "number" is only a relative expression of how the bullet a person is measuring compares to the standard model. Kind of like a datum line, the position of which is more or less arbitrary. So, form factor = coeff. drag / coeff. drag of the standard model. Simply, form factor is measure of drag and it is only ONE number in the calculating(give or take errors) of ballistic coefficients.

A multitude of model bullets/projectiles are used by people measuring the efficiency of a given bullet. Some models, for example, take into account the "aero" dynamics of the bullet at trans and super sonic speeds, while others do not. The number that is published for a bullet is simply a ratio as compared to the standard i.e. BC of .500 is half the efficiency of the standard model used. However, that .500 is relative to the testing methods and measuring inaccuracies used in deriving that .500 B/C number i.e. MY .500 B/C might not equal YOUR .500 B/C because we used different methods......mine would be better that yours though!

If you must know how they get the form factor "number", it is by firing a "standard" bullet/projectile, usually weighing one pound, measuring one inch in diameter, and about 3.2 caliber in length, measuring it's deceleration and giving it the arbitrary number of 1.00. Then they measure every other tested bullet and make a ratio between the two. i.e a Barnes 270 grain triple-X with a B/C of .500ish is roughly 50% as efficient as the standard it was measured against.

ANYHOW!! Like YOU said, it's pure snake oil!! Here's a link. This guy splains it better than I can. My feeling is it does not matter what the actual form factor number is, so long as it's measured precisely and you know what model it's measured against.

http://www.shootingsoftware.com/coefficients.htm

Cheers!

[Vindalbakken]

Not to mention the fact that for any given shape of bullet the BC varies with the velocity of the bullet.

[twofifty]

Originally Posted by pogo

Hmmmmm...

...first off, I'm no expert. I just find ballistics interesting. Like aerodynamics are interesting. Except with projectiles you just get one BIG, SUDDEN shot of horsepower at the beginning and then you are pretty much "coasting" the rest of the way!
......
http://www.shootingsoftware.com/coefficients.htm

Cheers!

An interesting subject that's for sure.

I read somewhere that naval artillery, such as the 16" battleship guns of WW2, fire their +/- 2000 lb shells at a what amounts to .308 Win muzzle velocities. So let's say 2,500fps. The shell 'coasts' out to 30+ km.

Kinda surprising that everything is scaled up, except for the velocity. Dunno how fast or slow the powder bags burn, probably slow as those are darn long barrels.

[sdeviation]

So,,, in reality the BC is a useless piece of information that is very usefull...
its just a guide line ,,and has nothing to do with accuracy ,and there r way to many variables to figure it out correctly but one can come close and have alot of fun figuring it out ..ie: click ,bang ,DOH!!!!!

[222rem]

Originally Posted by sdeviation

So,,, in reality the BC is a useless piece of information that is very usefull...
its just a guide line ,,and has nothing to do with accuracy ,and there r way to many variables to figure it out correctly but one can come close and have alot of fun figuring it out ..ie: click ,bang ,DOH!!!!!

No.......... BC is not useless piece of information

If you are long range shooter BC is not useless piece of information Bringing low BC bullets to a 1000yds shoot is like bringing Yugo to a Formula-1 racing .

[sdeviation]

Bringing low BC bullets to a 1000yds shoot is like bringing Yugo to a Formula-1 racing .
xcactly my point,,, low or high BC they will both make it there


the yugo will make it slowly but surely with little problems but the F-1 car has to be proven and trial driven ..it might over heat ,it might slip in a corner ,engine might blow at 13,000 rpm