Harmonics is easy to work around, as most harmonic problems in precision bolt-rifles are a narrow window that you can tune out. I've rarely seen harmonic issues in most precision rifles.
The heavy barrel guns help reduce the total harmonic flexure anyways, but it still doesn't take much flexure to induce problems if it is at the crown when the bullet leaves. You're talking a tiny, tiny fraction of a second of a difference, which can be mitigated by something as simple as seating depth and neck tension.
BC is nice, but it pretty much just dictates how well the bullet retains it's velocity during it's flight path. Bullets with poor BC's develop alot of drag, and that causes them to shed velocity and drop quickly. However, because the bullet travels at a fraction of a degree, due to gyroscopic forces from the rifling twist, the added drag can cause an increase in spin-drift (Magnus Effect).
That is also why match bullets have lightweight or hollow front nose construction, to polarize the centroid mass of the bullet to try and mitigate the magnitude of the gyroscopic cant.
