First, let me stipulate that I have no “applied” experience beyond the “Briefcase/Bugout Bag PDW” an LEO buddy and I cooked up.
Starting from front to rear:
[ol]
[li]Definitely some kind of concussion management system is in order–ideally a suppressor, but if your area is NFA-unfriendly or you don’t wanna fight with the ATF paperwork at least a blast shield like Ferfrans CRD, Strike Industries Oppressor, Fortis Control or Indian Creek BFD at minimum.[/li][li]Unless you have length restrictions, a 12.5" barrel seems to be about the sweet-spot from what I’ve read–handier than a fullsize M4, less beastly on blast etc. and better ballistics than a 10.3-10.5". In Stickman’s words, “the [12.5”] GL/SSC out-Mk 18’s the [10.3"] Mk 18.“[/li][li]When under stress and the adrenal dump hits, less moving parts to fight with is better–unless you need a magnified optic, I’m a big fan of fixed front-sight posts (keep all the moving parts concentrated on the back end), and for something that “discreet carry” or “low profile” are not a consideration might consider a fixed rear BUIS too. (Exception being eye trouble–I’m a big fan of Diamondhead sights due to my horrible eyes, and visual needs trump fixed/folding. You can ALWAYS leave a folder deployed…)[/li][li]I like longer handguards because of additional room to mount accessories, or to reposition them based on ergonomic needs.[/li][li]Short barrels tend to be harder on wear and reliability–definitely either a Geissele Super 42 or Vltor A5 buffer system. If you need a shorter LOP or have to fight with an OAL restriction, Geissele gets the nod since it fits into a standard CAR/M4-length tube, but if you have the room Vltor’s worth the extra 3/4”. (A5 is basically the original fixed-stock recoil system in a telestock/pistol tube and a shorter but same weight buffer.)[/li][li]If low overall weight is a concern, either Shockwave Blade or no brace. If balance is more important than OA weight and you like things tail-heavy, look to the SB braces and their mag-carrier and add-on bag accessories.[/li][li]Building on the prior, while it adds weight I’m a BIG fan of having a few spare mags mounted right on the weapon so you don’t need to worry about grabbing a carrier. [/li][/ol]
To elaborate on “onboard mag carriage,” there are many ways to do this. The oldest is to couple two mags together with clamps, or duct-tape 'em for a “drop, flip, shove in, reloaded.” CAA makes a rail-mount carrier designed for one of their stocks that works just as well on a fore-end rail. If you’re above 26" OAL, you can use FAB Defense’s mag-carrier foregrip; below 26", attaching this part makes an NFA AOW. As noted above, SB has a carrier that nestles neatly into the middle of their braces–in theory you could lash one to either side of a Shockwave Blade, but in practice Marty at Shockwave is concerned that any alteration might jeopardize the “not a stock” classification. So for a Kino or GL/SSC pistol, with one carrier on each side-rail, one in the foregrip and one in the brace, that’s four spares ready to go plus the one in the well–and if you use coupled mags everywhere except the brace, you’ve got 270 rounds on tap, more than the standard infantry combat-load (IIRC, only 7 mags). Taking this to a ludicrous extreme, if you have Hulk strength you could do a D60 drum in main and foregrip (120 rounds), coupled 40-round PMags in forearm carriers (2x80 = 160, total 280), and if you have a long enough arm to run the brace out that far, another 40-rounder for a final total of 320 rounds.
EDIT TO ADD: If you really want to get truly ridiculous, slap the new Odin Works 5-round extenders and couplers on. You might be able to squeak coupled 40’s onto the SB carrier, so adding +5’s that brings the Ludicrous Load up to 390 rounds–one USGI mag away from a full spam-can right there on the gun. Dumping the drums for paired-and-+5’ed 40’s, that would get you a whopping, just plain STUPID four hundred fifty rounds on the gun… but how do you expect to HOLD it? LOL
After I’ve had a chance to watch your video with audio (I’m in a sound-unfriendly environment as I type this) I’ll put up the 10.3 I have under construction and the 12.5 I’m penciling out in the name of sharing ideas and inspiration.