Quote Originally Posted by Forester
To extend the line of thinking though...that smaller diameter, same weight bullet should have a significantly higher BC. Thus it does not need to reach the same speed in order to have superior ballistics. Muzzle velocity is perishable but BC is forever.

I don't know who told you BCs are forever, 'cause it just isn't true! BCs are dependent on velocity. As a matter of fact that is it's definition....

"What is a Ballistic Coefficient?
: The ability of the bullet to maintain velocity, in comparison to a ‘standard projectile’. A high BC bullet can maintain velocity better than a low BC bullet under the same conditions. All measures of ballistic performance including drop and wind deflection are related to the bullet’s ability to maintain velocity. In short; the higher the BC, the better the all-around ballistic performance of the bullet will be."

In ballistic software, BCs are usually given at different velocity thresholds (but realize that the BC is decreasing as the bullet loses speed). Thus it is a constantly changing value.

Here is an example of a load I use out to 600 yards....

Gun / Ammunition : 6 mm B.R. Norma
Bullet : .243, 80, Sierra SPBT Blitz 1515
Bullet weight : 80 grains or 5.18 Grams
Muzzle velocity : 3179 fps
Crosswind speed : 1 Mph
Ballistic Coefficient(s) (G1):
C1=0.313@V>2800 fps;
C2=0.305@V>2200 fps;
C3=0.290@V>0 fps;

As you can see, the BC is highest greater than 2800 FPS and decreases as the bullet get further from the muzzle and is slowing down.