Neat idea - I never thought to cast anything but a lead bullet - a semi-hard epoxy style resin might be good for tips if lead wasn't an option, provided the mold was well coated in releasing agent!
For the primer pocket I've poured hard rubber resin - no idea what it was but it came out the same hard consistency as the hard rubber mouth guards - they don't mark at all but rebound the hammer nicely - They tend to pop out eventually unless there's a little bit on the far side of the primer pocket to anchor it.