Effect of Ballistic Coefficient

[MatthewG]

I used a formula from one of Lilja's [great] articles to determine relative bullet BC as a function of temperature, pressure, and altitude. I am wondering how the BC gets further implemented into a formula?

Eg. I am shooting at 200yds and know my .243 80gr .360 BC Fowler is normaly on center, but on this day the temperature is 70 degrees instead of the normal 59 and I am at 5000 ft altitude instead of 0, thus changing my BC from .360 to .373. OK, but how do I use that?! I understand impact will rise because the bullet can get through the air more efficiently, but mathematically where do you use BC?

Thanks!

 

[Asa_Yam]

... but mathematically where do you use BC?

In a ballistics program. ,Sorry couldn't resist.)

Seriously, a "ballistic coefficient" is a conversion factor used to predict performance of a bullet using data gathered from ballistics of a "reference" projectile. Before personal computers were widely available, it took a well-equipped,usually government) lab to collect the data needed to generate an accurate ballistics table. Once you had the data, it took more than a few people using slide rules to convert it into a usuable form. If all you have to do to compute trajectory of a new projectile is multiply results from the master table by a scale factor, the calculation is greatly simplified.

There are multiple ballistic models, each using their own reference projectile. The model used by 99% of ballistic models is known as G1, and it usually works well out to 600 yards or so. Beyond that, predicted results usually diverge from observed, because G1 usually isn't an accurate model for the types of bullets normally used at such distances.

For more on the subject, see http://www.exteriorballistics.com/ebexplained/index.cfm and http://bryanlitz.bravehost.com/BalProgs.html

A good online,i.e., free) ballistics calculator is available at http://www.eskimo.com/~jbm/calculations/calculations.html