The primary component in the BC is the Sectional Density or SD, which in any given caliber means weight.... the heavier it is, the better the BC.... The other component in the BC is the "Form Factor", which is related to the drag of the particular shape involved.... They are all related by the formula....

BC = SD / FF

In a given caliber, if you keep the weight constant, you have a fixed SD, so the only way to a better BC is by lowering the FF.... In a nutshell, this means longer bullets, but if the weight is a constant, you then need a lower density.... Yes, you could make a Bi-Metal bullet, if you could control all the variables to make sure the core remained consistent and concentric with the OD.... but an easier way to experiment is just to cast some bullets up in pure Tin.... It has a density of 7.31, compared to lead at 11.34, so it's less than 2/3rd of the weight.... It isn't a lot harder than lead, and has a low melting point, so you can cast it in conventional bullet moulds.... If you used the mould for my 51 gr. BBT in .25 cal, you could end up with a bullet that only weighed 33 gr. instead of 51, but looked the same, and had the same FF.... Unfortunately, the BC would still only be 65% of the lead version.... so it would drop from about 0.11 down to about 0.07.... A 34 gr. JSB Heavy is about 0.053 for comparision....

As far as stability is concerned, the less dense the bullet, the faster you have to spin it.... Whereas my 51 gr. BBT is designed for a 14" twist, if you made it out of tin, you would have to use an 11" twist barrel to get the same stability.... So, yes, what you are suggesting is possible.... but you will need a MUCH faster twist rate for it to be accurate.... Tin is also much more expensive than lead, of course....

Bob