The possibility of using copper elongated projos in a sabot was brought up. One would think that matching the BC of elongated copper to lead ball would be easy enough- you need to do this to get the thing to stabalize in a slow twist barrel- copper is lighter than lead so matching the lead ball BC is just a question of length in copper. The sabot is another question. All I know of are made of polyethelene, and soft poly at that. Poly leaves residues in the barrel at the velocities round balls are usually shot at, which eventually gums up the bore, a combination of heat, friction, gritty BP residues doing the job. Also the poly will not expand to the depth of normal round ball grooves. They work well enuff in shallow groove inlines but not in the deeper grooves found in most RB rifles. And it is nigh onto impossible to force even soft poly into those deeper grooves even if you can find a sabot big euff to fill the grooves. So the situation, at least for now, is impossible. An alternative answer might be to use a cloth patched copper pickett style bullet, patched in cloth, but first you will have to purchase a 40 ton press and make the dies to produce them. Sounds expensive to me. Bismuth will mold, works pretty well in shotguns, I shoot geese with handloaded bismuth and get good kills, and I've never found a fragmanted bismuth BB in a goose, they go right through bone, the problem is they don't expand that I can see. I know of no studies on bismuth in larger calibers. Perhaps a tungston-matrix ball would work but you won't mold that at home. The shot is very expensive and I suspect the industrial effort for a small niche market would be prohibitive. I've brought up lots of negatives but let's not give up. Keep thinking. The answer may not be traditional but it will come. DOC
