Based loosely on advice from a friend who works in executive protection, I put together a 300 BLK pistol PDW for protecting my VIPs (family) while traveling by car. I can think of a few scenarios where a handgun wouldn't be enough. E.g. Armed thug roadblock, Katrina level civil unrest, etc. Since I wanted a very compact pistol length gun, I broke my general policy of avoiding adding more calibers to my arsenal. A 7.5" gun in 5.56 just doesn't make sense. But in 300 BLK, it works really well. This gun fits in a tennis racket case.
Palmetto "multi" pistol lower, Doublestar Strongarm brace, H3 buffer, Geissele ambi safety. The DS brace is aluminum, very solid, and is the most compact non-collapsing brace I could find.
Palmetto 7.5" 300 BLK upper, BCM BCG & CH, ALG 8" V0X Rail, TLR7, Trijicon MRO on Geissele lower 1/3 mount. (The upper came with a crappy MLok rail that I immediately scrapped.)
Lancer 20 round and Magpul 30 round 300BLK specific mags. (Note that I have verified that my 300BLK loads will not chamber in my 5.56 uppers.)
Barnes 110gr TAC-TX