I dont know enough about this rowland guy to quite understand all the hate thrown his way.
You didnt really address either my question about using .40cal cases or sabots. I understand that .40 has some drawbacks, however, like .357sig, it always has more energy that 9mm has to offer. The extra case capacity should allow a subcal bullet to reach higher energy levels, but without the need to form the brass and create a necked cartridge. Slap rifle rounds werent noted for their accuracy, which could in part be due to the wall thickness of the sabots they used, if they were not holding the bullet properly centered in the bore of the rifle. A thinner sabot should reduce that issue (potentially, Im no materials engineer).
And again addressing this threads circling back to .312 projectiles and .30 carbine as a caliber of choice.
If we look at light for caliber ammo choices for .40cal, we can look at liberty civil defense, which throws a 60grn schp at 2000+ fps. .30 carbine throws a 85-105grn pill at around 2100fps from a 16" barrel. So using a light sabot, and a .312 bullet in the 50-60grn range, we can see short barreled .30 carbine performance from a pistol length barrel, in an existing straightwall cartridge case.