You are doing it wrong!
I have been handloading my own shotgun sabot slugs (525gr) for the past 13 years!
I got my lead for free which means I can fire 525 grain slugs at 1500 fps for about 15-20 cents per round! That's <$1.00 for a box of 5!
All you need is this mould from Lyman:
A simple $35 Lee shotgun press:
Powder, 209 primers and wads are used as the sabots!
I have shot probably 1,000 of these slugs through my 870 with 20" rifled slug barrel, it will shoot sub 3" groups with open sights at 50 yards. :thumbup: