Assuming two bullets have the same exterior measurements, are of the ULD type design with RBBT, and both balance well. If one bullet weighs say 215 gr and the other weighs 240 gr, which would have the higher BC? Let's say these are .30 cal bullets traveling 3200 fps for the 215 and 3150 for the 240. I'm not trying to start an argument, just curious as to everyone's opinions. Thanks

wikipedia - ... "BC is a function of mass, diameter, and drag coefficient." It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in² or kg/m². BCs for bullets are normally stated in lb/in² by their manufacturers without referring to this unit..." or in other words, BC = sectional density/form factor

To put the above information into the context of your specific question... If they're truly the same shape, the 240 grain bullet will have a BC that's 1.116 times higher than the 215 at all speeds. Because 240/215 = 1.116. That's 11.6% higher BC for the 240 grain. So if the 215 has a BC of .665 at a given speed (or averaged over a given speed range), then the 240 will have a BC of .665*1.116 = .742. Bear in mind the 240 will suffer depressed MV, so the performance of the heavier bullet won't be as much better as the BC implies, but it will be a little better even at a reduced MV assuming equal chamber pressure for both bullets. -Bryan

Also, they won't be exactly the same shape if they are made from the same material/construction. The heavier bullet will be longer.

True, But if the same tooling is being used, the bullets will have the same nose and tail. The heavier bullet would only be longer in the bearing surface, and that has minimal affect on drag. -Bryan

There are ways to make bullets the way I described, even without any dead space in the bullets. Same jacket, same type of lead (just slightly different amounts), and just a little something extra in the lighter bullet.