Bullet coefficiency

BC will change with Velocity, the further down range the slower the bullet and the lower the BC gets. BC is the ability of the bullet to travel through. it will also chang with altitude sine the atmospher gets thiner the higher above sea level you are.
 
I was just trying to get a basic idea so I could input it into a ballistic program I dont have a chrono. so it guess work to get it close anyway.

Thanks, Stan
 
I use this calculator for my projections http://www.eskimo.com/~jbm/ballistics/traj/traj.html

Blindog is correct that your BC is variable on atmospheric conditions this is wth best explanation I can produce from wiki.

"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."

" Use of ballistics tables or ballistics software based on the Siacci/Mayevski G1 drag model, introduced in 1881, are the most common method used to work with external ballistics. Bullets are described by a ballistic coefficient, or BC, which combines the air resistance of the bullet shape (the drag coefficient) and its sectional density (a function of mass and bullet diameter).

The deceleration due to drag that a projectile with mass m, velocity v, and diameter d will experience is proportional to BC, 1/m, v² and d². The BC gives the ratio of ballistic efficiency compared to the standard G1 projectile, which is a 1 pound (454 g), 1 inch (25.4 mm) diameter bullet with a flat base, a length of 3 inches (76.2 mm), and a 2 inch (50.8 mm) radius tangential curve for the point.

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 1.00 being the most aerodynamic, and 0.12 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[1].

Sectional density is a very important aspect of a bullet, and is the ratio of frontal surface area (half the bullet diameter squared, times pi) to bullet mass. Since, for a given bullet shape, frontal surface increases as the square of the calibre, and mass increases as the cube of the diameter, then sectional density grows linearly with bore diameter. Since BC combines shape and sectional density, a half scale model of the G1 projectile will have a BC of 0.5, and a quarter scale model will have a BC of 0.25.

Since different projectile shapes will respond differently to changes in velocity (particularly between supersonic and subsonic velocities), a BC provided by a bullet manufacturer will be an average BC that represents the common range of velocities for that bullet. For rifle bullets, this will probably be a supersonic velocity, for pistol bullets it will be probably be subsonic. For projectiles that travel through the supersonic, transonic and subsonic flight regimes BC is not well approximated by a single constant, but is considered to be a function BC(M) of the Mach number M; here M equals the projectile velocity divided by the speed of sound. During the flight of the projectile the M will decrease, and therefore (in most cases) the BC will also decrease.

Most ballistic 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 flat-based, spitzer, boat-tail, very-low-drag, etc. bullet types. They assume one invariable drag function as indicated by the published BC. These resulting drag curve models are referred to as the Ingalls, G1 (by far the most popular), G2, G5, G6, G7, G8, GI and GL drag curves.

How different speed regimes affect .338 calibre rifle bullets can be seen in the .338 Lapua Magnum product brochure which states Doppler radar established BC data.[2] The reason for publishing data like in this brochure is that the Siacci/Mayevski G1 model can not be tuned for the drag behaviour of a specific projectile. Some ballistic software designers, who based their programs on the Siacci/Mayevski G1 model, give the user the possibility to enter several different BC constants for different speed regimes to calculate ballistic predictions that closer match a bullets flight behaviour at longer ranges compared to calculations that use only one BC constant.


You can also take into consideration other things in long range shooting to include Coriolis effect which will affect the trajectory due to the rotaion of the earth at long range.
 
I insert the BC listed by the manufacturer into the JBM chart. Then after inserting good chrono data, bullet weight, etc,....the chart can be run first. Next,..you take the target out several hundred more yds and measure your drop and compare it to what the chart said it should be at that distance. If it is not the same,..you gradually change the BC figure and re-run the chart until the chart impact matches your real world impact. Now you have a chart that will be off very little when conditions change,..even at 1000yds it won't be off by more than a few clicks.
 


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