Here are some 20ga slugs/sabots we rigged up.
I traded for whole box(hundreds) of 20ga Hammerhead
slugs, in the picture. They had pulled slugs
out of the locked on plastic base, so I glue them
back in so base stays for fair smoothbore accuracy.
And I also glued in various 458 bullets, as shown,
which fit tight in cup, up to 500gr. They discard
on impact. Fair accuracy at 50-70 yds. Less expensive
shooting as I have pile of hard lead 458 slugs.

Idea on 8ga FH. Looked at a TCR87 break action 10ga
and the breach mono-block can take a little bigger
barrel.If I find one I can afford(pricey) I'll do 8ga.
These are very strong actions, a little smaller version
of the action setup Colin put the 2bore in.
Just put in straight 8ga bull barrel like NEF Ultra.ED