"Identical shape, identical amount of aerodynamic drag but still a different BC."
The two bullets jwp475 mentioned do if fact have dissimilar shapes. Even though the WC bullet is lighter it is also more aerodynamic. Thus the higher BC.
If you took the same size/shape bullet and magicaly increased its density to, say twice that of lead, the BC would increas accordingly.
Non of this stuff is magic, it all comes down to physics.
Mass x velocity = momentum Which means that two objects of dissimilar mass but equal velocity (assuming the size and shape are identical) will be effected by the forces of drag differently. The object with the larger mass will require more force to slow it down at the same rate as the smaller one, and since the force of drag will be identical for both ( ie, the identical shape and size) it will require more time.
Thus since it requires more time, it can be said to be more efficient, as far cutting through the air anyway. The more time it takes to slow it down the higher the BC.
I was replying to jwp475, on his request for an explanation on how a lighter projectile can have a higher BC.
He had mentioned the wildcat 265gr and the 300gr Matchking, which have dissimilar sizes and shapes.
But, what i said carries over to the original question Hogslayer had.
"80, 85 90 grn bullets all were the same jackets ,same length , just have more lead in the core. My question is does this change bc"
My answer was yes it does change the BC.
But, and there is always a but. Only if the bullets (the 80, 85, 90gr wildcat bullets) have the exact same profile as one another, and the only thing that changed was the mass, will the BC increase in the way i stated.
If the profile/shape does change, then the BC will also change, but not just because of the change in mass.
It is possible that the BC stays the same or even goes down as the mass increases, if the heavier bullet is less aerodynamic than the lighter bullets.
Oh! and my post should have read "IN" and not "if". (The two bullets jwp475 mentioned do "if" fact have dissimilar shapes)