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.
A bullet's B.C. is a ratio of its Sectional Density (S.D) to its coefficient of form (i), so B.C.=S.D./i. Coefficient of form (i) is dependent on a bullet's shape but is a constant for a given diameter and shape. Sectional Density is the ratio of a bullet's mass (m) to its diameter (d) squared, or S.D. = m/(d*d). In other words:
B.C. = S.D./i = (m/d*d)/i = m/(d*d*i)
You might notice that mass (m) is in the equation. As a result, while you can have different bullets of the same diameter (d) and same coefficient of form (i), but if their mass (m) is different they will have different B.C. values.
Compare a 3" styrofoam ball with a 3" lead ball. Same coefficient of form (i), same diameter (d), same shape, different mass (m) and very different B.C. values as a result.
So no, you cannot have two identically shaped bullets (including diameter, as in .264") with different mass that have the same B.C.
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.
Again you duck and dodge. A bullet that shoots flatter because of its B.C. also has less wind drift. But that difference only comes into play at longer ranges - B.C. is pretty irrelevant at close range. As I showed above, B.C is directly tied to a bullet's mass. Velocity is important to you else you could reduce recoil by shooting all your bullets at 100fps. They wouldn't be very effective, but they would have very low recoil.
As soon as you specify a caliber, B.C. and velocity you are also specifying a specific energy. Transforming the equation "B.C.=m/(d*d*i)" from above yields this:
B.C.*d*d*i = m
And we already know energy (E), mass (m) and velocity(v) are related as follows:
E=1/2*m*v*v
Therefore:
E=1/2*(B.C.*d*d*i)*v*v
As a generally reasonable person I expect you would not take a shot if the range is such that velocity falls below the range where, in your judgement, your bullet will be effective. For purposes of discussion let's assume 1800fps for the 123g .264" Scenar. (Feel free to pick a different number.) It is also safe to assume there is an upper limit to the velocity at which your rifle can launch such a bullet, say 3000fps? If those conditions are true, velocity does matter to you and we have a reasonable working range of velocities at which you would use that bullet. We also have a range of energy values at which you expect the bullet to be effective.
So while you may not think of it in terms of energy, if you specify a bullet by diameter and B.C. and there is in fact a minimum velocity below which you would not take a shot, there is also an easily defined minimum energy level at which you deem that bullet's performance to be unreliable.
As I stated early on in this thread, energy is an imperfect tool but a useful one nevertheless. People use it as a rough gauge because energy determines the maximum work (destruction) a bullet can do. The actual work (destruction) a bullet will do depends, as I've repeatedly stated, on a variety of factors including bullet construction and types and amounts of target material encountered.
In any case, it is certainly better than using velocity alone. Or B.C. alone. Or mass alone. Or S.D. alone. Or momentum alone.
