An AR in 5.56, especially short-barreled is a nearly ideal home defense weapon, except for one glaring problem, which makes it nearly unsuitable for the purpose: muzzle blast and noise.
You can easily do permanent hearing damage with a single shot from a short barreled 5.56 indoors.
Otherwise, they work great... precise, high capacity, easy to shoot, excellent terminal performance without over penetrating.
Supersonic, the .300BLK is just as unsuitable as a 5.56 for home defense for all the same reasons, with the additional problem of over penetration. With subsonic rounds, the .300BLK tones that package down to handgun levels. Basically, it's a pointy .45acp in a high capacity, easy handling package that is fantastic suppressed. Unfortunately, it's going to over penetrate compared to a handgun caliber. Not sure if anyone has developed a heavy .30 cal bullet that expands reliably at subsonic velocities.
IMHO, for the average homeowner, a handgun is easier to use for home defense, but if I were to go carbine I would go with a handgun caliber.