Over the years there have been wildcats made out of just about everything there is. Had a friend that had a 50 BMG necked down to a .17 cal bullet (as a gag). Have also seen 50 BMG with a sabot, driving a 30 cal bullet. Things is, the 50 BMG has about the best ballistic coefficent of any bullet (posible exception- some of the VLD bullets) You do run into an upper limit of speed, where lead bullets start to fly apart from the speed of the spin. Solid bronze or copper might work around that some. But then you have to answer the question- "Where the heck am I gonna shoot this thing?" Unless you have an artillery range, may be kinda hard to find a place to make go bang!