Well, I don't know if you could completely RUIN the B.C. of a bullet, but you can certainly damage it, and it doesn't take much of a change in the projectile's configuration, or even the slickness of the exterior surface, to reduce the B.C. noticeably.

In addition, some manufacturers have a tendency to claim higher ballisctic coefficients than their projectiles seem to have, when you test them (many are "calculated" based on shape,rather than determined empirically on an instrumented test range.....)