I have been reading this with interest
I like a .410 for upland game and ground quarry.
and a slug load like this would be a very handy addition to a versatile firearm.
I think I may just have to try this
as for drilling a hole centered, here is my thoughts:
using scrap wood, drill some test holes to find a bit that drills a diameter that "Just" fits the boolit
find a 1x2 of a decent hardwood, about 12" long, and cut a narrow kerf down the length, to about 1" from the end
now, clamp that board onto the drill press table
now drill your boolit diameter hole, about 1/2 inch up from the kerf bottom, so you have about 1/4 to 1/2 inch of kerf on one side and the ten inches on the other, to the end of the board, and centered on the kerf
now add some of those nitro cards into the hole, to build it up enough so when a boolit is dropped in, you can pull it out with fingers.
Now, change your bit to the one you need for your screw.
drop your boolit in and SQUEEZE the ends of the board together to "Clamp" the boolit and drill down into the boolit.
use the depth stop, or a bit collar to set your depth.
if it holds well, drill away.....