So far I have found no .410 slugs for sale! I have used a .375 lead ball in a cut down case so far, without crimping - I will be getting a crimp and will do some experimenting with .357/.38 and 9mm cast bullets.
There area couple of ways for the 410.
1. Use a 148gn HBWC in a cut down plaswad to suit. 11gns Red dot maximum
2. Use a 41 caliber swc bullet backwards - approx 200gn - with the nose glued into a 410 plastic cup then pushed into the case.