Balistic coefficent of 2 Lee 30 cal molds

[Marlin Hunter]

Lee lists the B.C. of the C309-180-R as .317 and the C312-185-R as .247. The .312 is .003 larger than the .309, which would account for the extra 5 grains in weight. Basically the 312 is the same thing as the 309, just .003 bigger, so why is the B.C. so different?

[243winxb]

http://www.uslink.com/~tom1/calcbc/calcbc.htmCoefficent is measured over a given distance. The fps at the muzzle will have an effect on the numbers. The C312-185-R is fatter, pushing more air so a lower number of .247 If you look at Sierra rifle bullets, they list 3 different coefficents @ different fps for the same bullet. If you find out why, let us all know.

[243winxb]

.240 @ 2800 fps and above
.280 between 2800 and 2200 fps
.330 between 2200 and 1500 fps
.355 @ 1500 fps and below-Ballistic Coefficients for Sierra 180 RN

[mpmarty]

Ballistics is a fascinating study. Internal vs. External ballistics is quite a subject. The better the interior ballistics the worse the exterior become. Sectional density and shape determine the ballistic coefficient of a projectile such as nose shape, boat tail etc.:coffee: