Hijack - Any wildcat round has tradeoffs and benefits. Personally I'm not a fan of 5.56 for defense, it's a fast little round great for coyotes and other short-medium range small targets. The .223/5.56 round was never designed for short barrels, and it's a waste in them (more boom, less velocity), as shown at
Barrel by the Inch. Since you're running at those velocities anyway with a short barrel, why not use the powder to push a bigger caliber, like a .308 round. Cutting off the scrawny neck and putting a .308 round in the same cartridge to form the .300 blackout wasn't a bad idea. It's bigger caliber so bigger holes. It doesn't run as fast so you can actually suppress it, less powder means you can use shorter barrels without all the noise. That comes with a range trade off, so you won't be shooting one 500 yards. But then again how many people actually shoot their AR 500 yards to start with? IMHO 500 yards is a range at which you need to be using a more traditional hunting rifle with good magnification, not a semi-auto loaded with a flashlight, laser sight, reflex sight with magnifier, and a foregrip. A pair of pliers makes a poor hammer, use the right tool for the job.
I like 300BLK in idea, but in reality, the ammo is hard to come by and expensive. So instead of some wildcat chamber I have a 9mm AR instead. It's no 300BLK, but it's good for anything I'd need to shoot inside of 100 yards. Ammo is cheap and easy to come by, there's always a few thousand rounds in my cabinet to feed pistol practice too. The gun is just for fun, because honestly I'm not patrolling the sandbox for insurgents, or the jungle for Charlie. Home defense involves pistols and an 18" barrel shotgun, if I need more rounds than those can provide, I'm living in the wrong part of town or pissed off the wrong person!