Another homogenius bullet is a paper patched cast bullet. The hardness of the bullet can be adjusted by the alloy. If you want hard, use linotype. Not hard enough? Try anti-friction babbit at about 40 BHN. That's the equivalent of copper. These hard cast bullets are essentialy solids.

If you could get an X bullet at bore diameter and patch it up to groove diameter it might be safe in an older double. I think it was Whitworth who shot patched heat treated steel bullets without damaging the barrel. Even if you can get an X bullet down the barrel, what about regulation? The length-diameter ratio is so much different than a lead core bullet, that it must alter the point of impact.

Dave