This picture of an RC4 brand slug I see is available
down under. It has locked base like Brenekke.
And locked on without discarding petals, like US-S.

And 2nd picture is a group fired with RC4 slugs
by fellow on NitroExpress forums from
double 12ga, 2 left and 2 rights at 25yds.
I show this to promote the use and testing of smooth
bores for accuracy with right slugs as opposed to
to neglecting the possibilities they have.

3rd picture is a discarding sabot base, and with
four discarding sabot carriers, with centered
penetrating solid bullet, from France.