Best subgun cartridge in my humble opinion is the 10mm. But it is another barely alive caliber, and options are few, even though it IS catching on a bit here and there. I shot an MP-5/10 many years ago, FBI gun, and with two round burst it was very controllable. Don't think we'll see the like again.
Now imagine the Scorpion in .224 BOZ, or 5.7x28mm.....