First is picture sectioned Explora case and slug, showing how
they kept weight down so that it could be stepped up in
speed in older doubles, with just short rifling section at muzzle,
that didn't have real heavy barrels. Second picture from Greener
book here, is of older 4 and 8 gauges.

Third picture shows steps of making 4bore case, one inch bore,
from 20mm. 2nd in picture is case with base turned and swaged
to size which leaves base smaller than rim, for rimmed case.
Then 3rd case has top expanded to take 1" diameter slug.
last shows the thickness and strength of finished case.

Testing factory 20ga Hastings sabot slug 3.5" hotrod loads, and
reloaded with 20ga SPW identical slugs, 390gr, goes about 2200
from 30" heavy barrel. Load was 75gr of 4759.
Factory is about 1900. For regular modern barrels you could
do with 65 gr and get about 2000.Ed