Ballistic coefficients of Bullets question

[uplandhunter870]

im trying to get into the reloading rifle brass game and have been doing a bit of research on it but i have a question i cant seem to find "the" answer for.

so can anyone please explain to me what the ballistic coefficient of a bullet has to do with anything. and i have a pretty heavy science background so dont be afraid to use big words if you have to

Thanks in advance,
Steve

[washelkhntr]

Ballistic coefficient (BC) of a bullet is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant (I)  that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BC's are stated in lb/in² by gun projectiles producers without referring to this unit.


That answer it for you?

[high country]

or real simple like...the higher the number, the "slicker" the bullet

[carpsniperg2]

washelkhntr hit it on the head and then some.

[demontang]

Bc is basicly how well the bullet can keep its speed. the higher the number the farther it can go with less drop.

Your going to see a change in the BC stuff soon. They are starting to switch the boat tail bullet bc numbers fromg g1 to g7. The g1 bc used a profile of a flat based bullet to get the bc and the g7 uses a boat tail profile. so take the .338 accubond, its bc now is .550 with the g7 math it will be .273. The new g7 is more accurate for people that shot long distance.

[high country]

also it is not a static number.....varies with velocity.

[hookr88]

There is more info on BC at http://www.chuckhawks.com/bc. I was just reading it before I saw you're post and thought it might help.

[Curly]

Ballistic coefficient (BC) of a bullet is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant (I)  that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BC's are stated in lb/in² by gun projectiles producers without referring to this unit.


That answer it for you?

Interesting.  I always assumed BC was unitless.  Learn something new almost every day.

[Buckmark]

Also, a bullet manufactures published bc may be different than what you come up with if your using a computor program to calculate it yourself.
Berger bullets for example say there bc is +/- 1% accurate and a computor program may only be +/- 10% accurate.. 

[Intruder]

One of the complaints you hear from long range shooters is that manufacturers routinely overstate their BCs.  It's to their advantage to use a drag coefficient in the formula that gives a high BC even it may not be the appropriate one for the bullet.  I've never done it but the best way to calculate BC is to use 2 chronographs.  There's a formula that allow you to calculate an exact BC for your gun, at range, at particular speeds using this method. 

[uplandhunter870]

Thank you Washelkhntr and everyone else for your replies, you all answered my question and gave me exactly what i was looking for.

Thanks again
Steve

[washelkhntr]

Bullet performance
The formula for calculating the ballistic coefficient for bullets ONLY is as follows:


BCBullets = ballistic coefficient

SD = sectional density, SD = mass of bullet in pounds or kilograms divided by its caliber squared in inches or meters; units are lb/in2 or kg/m2.

i = form factor, i = ; (CG ~ 0.5191)

CB = Drag coefficient of the bullet

CG = Drag coefficient of the G1 model bullet

M = Mass of object, lb or kg

d = diameter of the object, in or m

This BC formula gives the ratio of ballistic efficiency compared to the standard G1 model projectile. The standard projectile originates from the "C" standard reference projectile defined by the German steel, ammunition and armaments manufacturer Krupp in 1881.[3] The G1 model standard projectile has a BC of 1.[4] The French Gavre Commission decided to use this projectile as their first reference projectile, giving the G1 name.

A bullet with a high BC will travel farther than one with a low BC since it will retain its velocity better as it flies downrange from the muzzle, will resist the wind better, and will “shoot flatter”.

When hunting with a rifle, a higher BC is desirable for several reasons. A higher BC results in a flatter trajectory which in turn reduces the effect of errors in estimating the distance to the target. This is particularly important when attempting a clean hit on the vitals of a game animal. If the target animal is closer than estimated, then the bullet will hit higher than expected. Conversely, if the animal is further than estimated the bullet will hit lower than expected. Such a difference in bullet drop can often make the difference between a clean kill and a wounded animal.

This difference in trajectories becomes more critical at longer ranges. For some cartridges, the difference in two bullet designs fired from the same rifle can result in a difference between the two of over 30 cm (1 foot) at 500 meters (550 yards). The difference in impact energy can also be great because kinetic energy depends on the square of the velocity. A bullet with a high BC arrives at the target faster and with more energy than one with a low BC.

Since the higher BC bullet gets to the target faster, it is also less affected by the crosswinds.

Sporting bullets, with a calibre d ranging from 0.177 to 0.50 inches (4.50 to 12.7 mm), have BC’s in the range 0.12 to slightly over 1.00, with high being the most aerodynamic, and low being the least. Very-low-drag bullets with BC's ≥ 1.10 can be designed and produced on CNC precision lathes out of mono-metal rods, but they often have to be fired from custom made full bore rifles with special barrels.

Variations in BC claims for exactly the same projectiles can be explained by differences in the ambient air density used for these BC statements or differing range-speed measurements on which the stated G1 BC averages are based. The BC changes during a projectile's flight and stated BC's are always averages for particular range-speed regimes. Some more explanation about the transient nature of a projectile's G1 BC (it rises above or gets under a stated average value for a certain speed-range regime) during flight can be found at the external ballistics article. This article implies that knowing how a BC was established is almost as important as knowing the stated BC value itself.



Differing mathematical models and bullet ballistic coefficients
Most ballistic mathematical models and hence tables or software takes for granted that one specific drag function correctly describes the drag and hence the flight characteristics of a bullet related to its ballistics coefficient. Those models do not differentiate between wadcutter, flat-based, spitzer, boat-tail, very-low-drag, etc. bullet types or shapes. They assume one invariable drag function as indicated by the published BC. Several different drag curve models optimized for several standard projectile shapes are however available.
The resulting drag curve models for several standard projectile shapes or types are referred to as the:

G1 or Ingalls (by far the most popular)
G2 (Aberdeen J projectile)
G5 (short 7.5° boat-tail, 6.19 calibers long tangent ogive)
G6 (flatbase, 6 calibers long secant ogive)
G7 (long 7.5° boat-tail, 10 calibers tangent ogive, preferred by some manufacturers for very-low-drag bullets[10])
G8 (flatbase, 10 calibers long secant ogive)
GL (blunt lead nose)

Probabally a little more than what you wanted, but what the hell, one can never have to much information.

[ADAMS]

Wikipedia is great tool, as washelkhntr will attest.

"... is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i  that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BC's are stated in lb/in² by gun projectiles producers without referring to this unit."

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

[washelkhntr]

Yep