Using other than pure lead for a full bore conical or PRB is setting yourself up for problems. I recently discovered this personally. You can alloy the lead if you are going to use a sabot or, as at least one of our forum members uses, a paper patched bullet with good results. The problem with using too hard a bullet is that the lack of expansion at the lower velocities. This is one of the reasons why saboted pistol bullets are used a lot. Because they are designed for similar velocities.