The only real advantage that going to a pistol caliber brings is reduced recoil. Given that 5.56 recoil is not really all that bad, and you can't take advantage of the reduced recoil with full auto or burst fire... there's just not a lot of point.
Now, if you could do it with radically reduced weight, that's something worth considering. That's why I actually think "Glock in a stock" has some merit. The KPOS on a G19 is 3 pounds. That is damned light, and it's one reason I think Gabe Suarez should have looked harder at them for his "SMG as EDC" concept. Also another reason to love the Sub-2000.
Now, if you could do it with radically reduced weight, that's something worth considering. That's why I actually think "Glock in a stock" has some merit. The KPOS on a G19 is 3 pounds. That is damned light, and it's one reason I think Gabe Suarez should have looked harder at them for his "SMG as EDC" concept. Also another reason to love the Sub-2000.