From what I understand, 300 BLK has a narrower use case than the hype.
300 is most suitable when a suppressed subsonic load is desired and a long-range option using only a mag change to supersonics is useful.
If only a subsonic load is to be used a suppressor, 300 is inferior to larger caliber loads with heavier bullets -- possibly even 9mm or 45 ACP. For a subsonic load, why limit it to 30 cal? The only reason is for quick-change back to a long-range option.
If going supersonic and above hearing-safe, the 223 performs better, and so does 6.5 Grendel, 6mm ARC etc.
So the only place I can see 300 being ideal is when both sub and supersonic are wanted without anything but a mag change.
Now this is what I've understood, but here is my dilemma:
Suppose I want a semi-auto gun that's suppressed and hearing safe. What's the ideal solution? It seems like pistol cartridges makes sense. A 45ACP will drive a heavy, large caliber bullet to nearly supersonic velocities, especially out of a longer barrel, so what's the need for a bigger case? The problem I see is that most PCC are blowback, which I hate. I tried a SIG MPX (9mm) and the reciprocating mass sucked. So what? MP5 or Kriss Vector? The HK is mostly unattainable and the Vector has its own weird recoil impulse (downward). The bottom line is the AR is not only a far better value, it's far better period. The only thing to like about the currently available PCC's is that they shoot cheap 9mm ammo. Otherwise, they suck. The brace rule did/could really jack up guns like the Vector and B&T too -- unlike the AR, they were never designed as rifles.
300 cannot be the best suppressed AR cartridge, unless you want to be able to switch to supersonics or just switch-out the upper for 223. Otherwise, larger caliber cartridges would seem to make more sense. But which one? 450 Bushmaster? I hate that it headspaces on the case mouth. 458 SOCOM? I don't know. This is something I don't know about.
FWIW, when faced with the question in the OP, I went 5.56x45, not because of the cheap ammo, but because of performance (without regard to a suppressor). If I understood the answer to suppressing to hearing-safe levels, I might do something different, but I don't think 300 BLK is the answer.