In your .260, for example, you specified a 123g Scenar, chosen for its accuracy and "decent B.C.". Given the caliber and shape, the B.C. is inextricably tied to bullet mass.
No, you're wrong. BC is not inextricably tied to mass. You can have two bullets shaped identically, with the same BC, but with different mass. One way to do that is to make the bullets out of something lighter than lead like the Nosler E-tip. The other way is to have a big cavity in the nose of the bullet, like Berger bullets and Scenars do, which increases BC but not mass.
Now, why do you want a high B.C.? High B.C. values allow a bullet to retain velocity and shoot flatter. In doing so (retaining velocity) they also retain more energy.
There you go again, telling me why I choose my bullets and getting it wrong, once again. That's not why I want a bullet with a high BC. As I said in my previous post velocity and flat trajectory are not all that important with a good repeatable scope that can be dialed for elevation. If velocity and energy were important to me I wouldn't be shooting cartridges based on the .308 case.
The reason I want a high BC is to buck the wind. Bullet drop is easy to compensate for because you have good numbers for all the variables needed to peg it--muzzle velocity, BC, range, and altitude (actually barometric pressure but altitude can substitute). Wind drift is much harder to compensate for because it requires an accurate estimate of the wind all along the bullet's path which is very difficult especially in tricky terrain where wind speed (and direction) will vary along the path. Estimation of wind speed is a huge source of error and a high BC minimizes wind drift making the error less significant. So minimizing wind drift is important and minimizing bullet drop is not.
I want a high BC bullet to minimize wind drift.
