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Shot does not disperse any where near as much as commonly believed. It still has to be carefully aimed. And the significant recoil makes it much harder to accurately shoot following up shots.
Burglers fleeing with the sound of racking the slide is a well disproven myth.
Buck shot, the standard defense load, easily penetrates walls, and significantly moreso than .223 ammo. Bird shot is not a reliable stopping load.
AR-15 style (modern) rifles with .223 caliber ammo is absolutely NOT ideal for 'distance on a battle field'. Larger caliber rifles are taken along for distance.
But where you really show your firearm ignorance is how you propose locking away a firearm and being able to use it easily. If you're planning on relying on a firearm for defense, practicing at least several times a year is a must, and cleaning and maintenance is needed for every type.
My point is that it's impossible and stupid to mandate the 'best' fire arm for defense. That's like mandating the best form of birth control. What if I have arthritic hands and I couldn't hold on to a shotgun's first shot?