the 300 BLK is a good round that has a little more power than a 30-30 in the AR15, there are many other rounds available for it. These are great self defense rounds.
There are carbines in 9mm, 40, and others. The advantage of a carbine, or rifle, is they are going to be easier to be accurate. You have more points of contact with the body. The shoulder, the cheek, the hand on the grip and the hand under the barrel.
We will prob not be in a breakdown of society, but best to be prepared.