It depends on need and context. For the most part I think most people are well equipped with a good 5.56 AR for self defense purposes. The caliber is capable of penetrating soft body armor with some minor exceptions. It shoots fairly flat and has decent terminal ballistics balanced with capacity,recoil,availability and reliability as well as accuracy. No caliber or rifle package is perfect for every task but the AR in 5.56 is pretty decent. If the shooter was wearing rifle rated armor the caliber along with actual cartridge chosen may make a difference but just alternate targeted area could work without changing caliber or cartridge. No need to go use 30.06 AP just change to upper body,head and neck or pelvic shots or side shot if available. The bigger more powerful calibers/cartridges give more options but have their drawbacks too.