... For example I'd say that a .223 loses out dramatically in the ballistics department at the range most HD encounters will take place. There is no way to accomplish the same amount of work with a .223 rifle in the same time frame that a shotgun can.
Between five shots of .223 and ten shots (centerfire maximum) of 00 buckshot and or slugs, what would be your choice?
Shooting someone with standard 00 buck is exactly like shooting them with 9 rounds of solid lead .380. Welll...380 has about 200fpe, each pellet from a shotgun has around 160fpe.
.223 has around 1500fpe at the muzzle, which is actually not that far from 160*9.
... even a reduced recoil load of 00 buck will have between 3-4 times the momentum of a 5.56 round. A full house load of 00 buck is between 4-6 times the momentum (Hornady's LM 00 buck load is 12 newtons if IRC). Slugs START at 11-12 newtons for reduced recoil and go up from there. ...
It comes down to inertia, but I'll expand it this way: as the mass goes up (e.g. you go from 3.5 grams of .223 bullet to 31.5 grams of 00 buckshot), both recoil and penetration increase for a given muzzle velocity. Obviously penetration is important, but it is also dangerous when it comes to passing through typical houses.
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I would recommend not basing your self defense weapon selection to any great extent on what happens if you miss your intended target. I'm not saying it's not a concern but it would be low on my list.
Please note, I'm not saying 5.56/.223 is a bad round, or that a carbine in that caliber is a bad choice. I'm saying don't count a shotgun out, as is just as effective in the HD roll.