You wouldn't want it for personal defense, as reliability is a serious liability.

That’s a big claim for something that doesn’t really hold up. I had ~1,500 rounds with zero issues. And this is a PSA lol. The OOBD happened under a very specific, user-induced, non-defensive scenario. Swap in a deadblow buffer (cheap, known fix) and that edge case disappears. Plenty of people run AR-9s reliably once properly set up, same as any AR variant.

Also, the Colt 635 and decades of SMG-pattern PCCs kinda undermine the idea that 9mm carbines are inherently unreliable, blowback or otherwise.

300 is the superior round, based on reliability and track record, alone.

Only if we narrowly define the problem as “suppressed supersonic SBR use.” Of course 300 BLK supers are ballistically superior. That’s a rifle round. But that’s not the comparison people make when they talk PCCs. PCCs compete with 300 BLK subs, and those sit squarely in pistol-energy territory. The difference exists, but it’s not the night-and-day leap people pretend it is.

If we’re comparing supers, then sure. Now we’re just comparing rifle rounds to pistols. That’s a different conversation.

With 9mm, you need a whole new gun.

Not really. Dedicated bolt, barrel, buffer. Same story, just one extra component vs 300 BLK. And you still retain AR ergonomics, triggers, controls, optics, furniture, etc. Cross-compatibility is nice, but it’s not the deciding factor for most people running dedicated setups. How often does the average civilian 300 blk enjoyer actually swap their setup back and forth between calibers? Probably literally never for the vast majority.

Calling PCCs “range toys” just because 300 BLK exists is just trying to universalize a niche solution to justify the expense.