Ok, this is an area where I waited a little too long, then jumped the gun, which, in retrospect wasn't that bad a thing. Something to keep in mind is that the 300BLK was designed for a shorter barrel (and subsonic with suppression) but retains much more kenetic energy at 100-200 yards (the general effective and intended range of a rifle caliber pistol) and uses essentially magnum powders or fast rifle powders, depending on what you are shooting. They are also much more reliable than the .223 due to the powder requirements. The .223 will still be loaded with slower burning rifle powder and will create a ginormous fireball and the round will lose about 30% muzzle velocity from the shorter barrel, while the 300BLK really only gains about 4% velocity for every inch past 8" of barrel length (for instance, my 10.5" barrel delivers only about 150fps less than a 16" barrel while a .223 loses almost 800fps from the same.
However, you reeeeeeeally want to reload for 300BLK - ammo isn't cheap unless you reload.
I just put together a 300BLK pistol:
View attachment 392441
The parts list from back to front and prices follow:
KAK Shockwave brace and tube $49 (ebay) --any pistol bufer tube will work but the KAK has little dimples for the set screw
Generic Carbine buffer and spring - $10 - Primary Arms
Anderson stripped lower - Rural King $49.98 (in store, no transfer)
Milspec LPK including fire control group - $36 - ebay
Stripped Anderson upper Primary Arms (they were out of completed uppers) $49
Some off-brand UPK - Primary arms $17 (together saved a whole whopping $3 over a completed upper...had they been in stock)
MilSpec MPI BCG - Palmetto State Armory $59.00
UTG "Sniper" Pistol Grip - $9.00 (ebay has them but I got one locally)
Angled 12" M-Lok hand guard - $52 - ebay **If you don't care for or want an angled hand guard, you can get regular keymod/M-Lok for around $25 depending on length
10.5" Melonite 1:8 mid-profile pistol gassed barrel - $72.00 Two Testicles Tactical
Generic low profile gas block - $14 (locally but ebay has them everywhere)
generic melonite gas tube $7 - local (again, ebay has them as well)
Krinkov Blast Can muzzle device - $18.00 ebay (I've changed it out for a slightly shorter one already and have it just lying around now...hint hint)
MOE M-LOK Hand Stop $12 - ebay
I think I paid $8 for shipping for the Primary Arms stuff and $0 for everything else
Grand Total: $462.00
Optics are on you

but I payed $18 for a set of aluminum flip-ups on Amazon a while back.
I went that route because nobody had exactly what I wanted (I
really wanted that angled hand guard and the flash can I ended up with) and certain things just weren't in stock anywhere.