Here is some more older cartridges and ideas.
First is a fellows collection of older brass cases.
They look like Berdan primed. Run from
36ga to 12ga, big one on right is flare cartridge.

Second is a paper case clad with
thin brass nearly to the top.

Third is picture of paper case crimped into
the side of a slug with shallow grooves.I did one
pictured above with a Dixie slug.

Some experimenting with 12ga discarding style sabot idea
that takes 58cal slugs/bullets.I pictured them above.
A fullbore,bore fitting sabot that takes 575-580 cal
slugs or bullets, cast or jacketed.

For reloaders to use, and not undersize ones
that have been available, for reloading guys. That
the guys haven't been able to get speed and accuracy
at the same time. These sabots are super accurate in
most tests, in factory Remington sabot loads. Much
more so than the undersize sabots reloaders have
had to work with.

I tested with the 440 gr .575cal hollowbase
Dixie Gunworks # BA-1103 lead slug and 3 shots from
heavy long barrel NEF nearly into same hole, at
30yds. Gun locked down. Going about 2200.
Our slow powder loads in 3.5" plastic.
Can be loaded faster.

No tests at long range, bad eyes/no scope, colder
than witches ---, and no time.
I am trying to find who makes these.There are
about 2000 injection mold companies in US