Ballistically, it's my understanding the 5.56/.223 may be good for both shorter and longer distances vs. pistol caliber carbines, but this is just more my ignorance or what I remember being told.
If anybody out there might elaborate, I'd appreciate it.
Reliability and lethality are two big factors. There are not a lot of shootings with 9mm PCCs to look at, in terms of effectiveness between a 9mm handgun and a .223 Carbine. Most ballistics experts will argue about the nuances of caliber stopping power effectiveness. But in reality, there are a lot of variables and when the practicality is looked at, a .223 with a proper bullet is about 3 times more effective at stopping a threat than a 9mm handgun (all things being equal and not a perfect handgun shot). When you add 30% more energy to that pistol bullet, you don't close that gap, but you do cut into it. When you add in the factor that a PCC is, on average, about 4 times more accurate than a 9mm pistol, you again, cut that gap, but still not closed. The sub-gun training, by most departments was sub-par.
Blowback 9mm PCCs, in general, are less reliable than .223 Carbines. They also need more cleaning and maintenance.
From contact to 100 yards, I have very close to the same accuracy and rate of fire from a .223 Carbine and a 9mm Carbine. Past 100 yards, the 9mm falls off pretty fast, while the .223 is going to be effective past that distance. But, if you look at the engagement distance of urban police shootings, very few are past 100 yards. The 9mm offers less concussion indoors as well. Trade-offs for sure. But I believe the PCCs of today are more accurate, reliable and effective than the sub-guns of the 1980s and 1990s, by a good margin.