BC are tested and derived in a specific set of conditions. They are property of the bullet dynamics, and those alone. When you work with ballistic models, they eat the BCs as an input, and "true" the output in your current conditions. BCs are not a static number, they're a dynamic variable. This is an in-flight rating of a bullet - whatever happens before it - isn't accounted for. Whatever length barrel, whatever twist, whatever rifling will not raise or lower a BC.
Bryan Litz's books are a great resource in long range precision/high power rifle. Compared to most books on projectiles, it is an easy read yet credible. (Litz is the fucking man)
http://www.amazon.com/Applied-Ballistics-Long-Range-Shooting-Understanding-ebook/dp/B00KRJXVZU/ref=sr_1_4?ie=UTF8&qid=1416610887&sr=8-4&keywords=bryan+litz