There is the 7.5x28 round developed by BRNO that gets you to within ~10% of .30 carbine power out of a 6" barrel. It's also just a slightly lengthened and necked down 10mm.
If you really want to pierce armour you'll have to go the sabot route. Take a .475 Wildey magnum and load it up with m80A1 in a sabot and blast it at ~2400 fps out of a pistol and ~2650fps out of a smg.
I really want a smg in these ridiculous auto-magnum calibers.