Setting the bastard pistol/tactical rifle combination aside, the .300 Blackout seems like the ideal round to replace the 5.56x45mm. All the myriad advantages of the AR/M4 platform, with the stopping power of an AK.
Now, as to the "pistol" tested, I would have to make a couple mods. Would need a laser sight, and an on-barrel video camera hooked to a "Google glass" or some similar video display. Then you could use the arm brace to good effect, firing "robocop style".

All kidding aside, that would let you fire from cover and around corners without exposing anything but your arm.
For sheer fun, I would need two of these critters, with 100 round double-drum magazines. Forget actually hitting anything- having one strapped to each forearm would kick ass!..
