Pure lead casts better from the Lee Slug mould. Use of w/w metal will give poor release from the mould.
On the other hand, I've gotten excellent results casting buck-shot from w/w metal. It's somewhat harder than factory pellets, even the plated ones and gives superior patterns.... Very tight if you slightly reduce velocities.
I've got a 10-pellet #1 buck load for the 20ga (.310") that will shoot 90%+ patterns at 40yds from a modified choke.. Typically 10" pattern at 25yds.
You can't get this performance from factory ammo, and no such load exists either.
With the slugs, I got poor casting from the Lee mould as the hollow base plug will "pull" plugs from the "partition", resulting in poor balance and un-even weights....
Besides, I've never gotten decent accuracy from the Lee "key-drive" moulds. I suggest you use the Lyman "sabot" mould that resembles an air-rifle pellet and is also loaded in a shot-cup (ie: Win AA-wad). This slug works equally well from a rifled or smooth barrel....