Jeff,
I've been casting slugs for about 15 years. Tried the Lee slug a few years ago and never got it to shoot well. The best I have found and the one I still use is the Lyman slug, looks like a huge air rifle pellet. I cast mine using straight wheel weights, would rather have complete penetration than expansion. Loads just like the Lee slug, drop it in a shot cup and fold crimp. Load info is in the Lyman shot shell reloading book. I get 3" groups at 100 yds out of the two slug guns I shoot it in, a mossberg 695 and a NEF Tracker II.