Originally Posted by Scott Gags

If the bullets have the same shape the heavier bullet will have a higher BC. The formula is as follows:

BC=SD/Form Factor

BC is Ballistic Coefficient
SD is Sectional Density
Form Factor is a representation of the shape

Since the Form Factor is the same in your example the only variable is SD. So the BC is directly affected by the SD and if the SD goes up by 10% so does the BC go up by 10%.

I believe a lead bullet is 27% denser than copper so a lead bullet of the same shape will have a 27% higher BC.