PCC for home defense
PCC's or pistol caliber carbines are becoming very popular recently and for good reason. Sure, they don't have the energy that a rifle cartridge has nor the reach that a rifle cartridge has. Many defensive situations do not require the reach of a rifle cartridge. PCC's can fit the needs for many defensive applications. For civilians, in most situations you would be hard pressed to argue your use of justified force in self-defense if having to shoot a threat at 100 yards or even 50 yards. The prosecuting attorney or judge would likely question why you didn't practice avoidance in that type situation if the threat was that far away.
Pistol cartridges can be a great home defense tool because most homes have rooms that are 15 feet across or less. If you are worried about possibly wounding other people in your structure or share adjacent walls with neighbors, the 9mm would also have less penetration than most rifle cartridges. The ATF reversed their stance on the classification of Pistol Braces in 2017, now calling pistol braces an accessory. This means they can be shouldered without any legal repercussion as it does not mean the user is redesigning the brace into a stock. 9mm reaches most of its potential velocity by 4.5 inches. In most cases the user might see at the most a 100fps increase in velocity from a 4.5 inch barrel to a 16 inch barrel. This means that a PCC has the potential to have a very short overall length from the combination of being able to use a very short barrel and an adjustable pistol brace. Whereas a 5.56 should be at least 10.5 inches in most cases to get the most reliability out of the rifle or "pistol". 9mm produces much less noise and concussion than most rifle cartridges. If you are going to be doing a lot of classes that require shooting in shoot-houses, the 9mm would cause less cumulative hearing damage. Also, the last advantage I will cover of pistol cartridges is the price of ammunition. Many pistol cartridges cost about half that of many common semi-auto rifle cartridges. This means you have the potential for more practice with your weapon, or for lack of better words, more bang for your buck!.
The trade-offs of using a pistol cartridge is that it does have less reach and much less energy than rifle cartridges. For most civilian defensive applications this isn't an issue as it will likely be used 15 feet and in. Pistol caliber carbines are a blow-back operated as opposed to rifle cartridges using a locked, rotating. This means that even in 9mm most PCC's have more felt recoil than even a 5.56mm. This is because there is no true lock-up of the bolt. It uses the energy from the cartridge to slide the bolt straight to the rear, then slides back forward and is held closed solely by spring tension. Some PCC's have much heavier bolts or a rotating delayed-blowback design to minimize the pop that PCC's tend to have.