I could be wrong on this, but I think the Barnes varmint grenade is a copper jacket with a tin alloy center. I suppose you could have copper jackets with an aluminum core or a ceramic core. Aluminum on steel may scrape the steel or fill it up with aluminum fouling after only a few shots, I do not know for sure. As as been pointed out, ceramic is very brittle and hard, it would have to be coated in some thing. It would have to be thick too, because ceramic will not conform to the rifling with out breaking up, so the coating would have to do this. It could be cost prohibitive too.