Modern, A nine is only effective at short ranges. From a practical standpoint since the conditions of use may be variable and undetermined, a firearm that can be used in qbc or longer range engagements would provide security in broader scenarios.
In regards to over penetration, frangible hollow points can limit penetration in 5.56 just as it does in 9mm.
A 9mm has about 47″ of bullet drop at 200 yards complicating aiming. A 5.56 had about 2.5″ of bullet drop at 200 making hits much easier. A 5.56 can still be effective out to 600 meters.(dependinwho you believe)
