Some more slug setups and ideas.

First is USS brand sabot that European cartridge has, without
locked base, a discarding 12ga sabot, with .629" brass slug.
It is as accurate in rifled barrel as its locked on base
counterpart, in smooth barrels. Up to 750 gr brass slug
I tested. 3 shots in an inch, 30yds.
Shown with BPI powder cup seals.

2nd is the USS slugs with locked on base for comparison.
With corresponding length cases they were designed for.

3rd are heavy solid tungsten slugs, the smaller .580" cal and
1200gr, the bigger ones .660" cal and 2200 gr.
A guy in Southern MI made a few for me to check out. Now these
too hard to fire fullbore, damage rifling, must be in plastic.
These are just what the super heavy slug guys might like.

4th is a 1000gr 58cal tungsten slug in a 12ga REM style
58cal sabot. Notice how short it is for being super heavy.