I'm old school also Catpop, and I was also taught about the general co-efficiency weights as they apply to each common caliber rifle bullets.
Two bullets, one heavier than the other, being driven at identical muzzle velocities, will produce two different trajectories at the same distances. This is because the heavier bullet will maintain a higher velocity longer than the lighter bullet, thus the heavier bullet does not slow down as quickly as the lighter bullet. thus it produces a better BC.
But there is a point in muzzle velocity, in which the lighter bullet, if driven at high enough velocity, it will produce a flatter trajectory, but only out to a certain distance. As an example, this is not an actual confirmed external ballistic estimate, as I don't have my ballistic charts in front of me right now. But for the sake of example, lets say a 40 gr. BT is leaving the muzzle @ 3900 fps, but a 55 gr. BT is delivering a muzzle velocity of 3500 fps., the 40 gr. BT will produce a superior, or flatter trajectory over the 55 gr. BT out to say 400 yards. But once those two BT's go beyond 400 yards, the 55 gr. BT will eventually reach a matched trajectory, as far how much it has dropped, that might be at 500 yds.. Then as the distance increases, the 40 gr. will have dropped more than the 55 gr. BT at the same distance.
Early on in my reloading hobby, I began to realize that the much higher obtainable muzzle velocity of a lighter bullet isn't always going to deliver the most desirable trajectory. Yes, out to a certain distance the lighter projectile is going to drop significantly less, but beyond that barrier, the heavier bullet becomes superior. And even though there are some high dollar range finders that deliver BDC estimates in either hold over, or are intended to correlate with the turret moa adjustment, it's absolutely essential to confirm all three, the load, range finder BDC estimates, and the BDC system on the optic in order to reap the full benefit of all those tools.
GS