OK, tagged for tomorrow reading over coffee and doughnuts...just coffee.

Quoted: Wow, I didn't know that "The AR15.com Ammo-Oracle" was the be all end all of ballistics information.   Seems to me ballistics is a subject that has been hotly debated ever since firearms were invented. I can think of some highly respected experts who would have a lot to say if they were told that an AR is better for home defense than a shotgun. Ever heard of Jeff Cooper?  The one who invented those 4 rules?  He hates the black rifle.  Ah yes, opinions are like assholes...

"Be all, end all" (isn't that supposed to be "End all, be all?") of ballistics information?  No, we never intended it to be.  However, on the subject of AR15 v. Shotgun v. 9mm/.40/.45 home defense we've done quite a bit of work.  That is because this argument comes up literally once every 60 days.  We wrote the Ammo Oracle for this express purpose, to collect the current "state of the art" of ballistics knowledge in one place so these sorts of arguments could be answered with a link instead of 25,000 words.  Clearly, we have failed since the number of arguments and their length hasn't abated one hair since we released the Oracle.  I suspect this is more a comment on the number of boneheads than the boneheadness of the Oracle, but I'm not exactly an objective observer in this matter.

For "home defense" given the choices outlined above we recommend the AR15 with M193 or better (75 grain/77 grain/69 grain/68 grain OTM).  We do so under the umbra of the following conditions and assumptions:

1.  12" penetration in calibrated gel is the minimum performance for an anti-personnel round.  Though a somewhat arbitrary number it seems to have held up as a good standard and though gel isn't perfect as a tissue simulator it correlates enough to good performance in tissue to be the best there is to offer.  Additionally, fragmenting rounds are preferred insofar as they open larger cavities and are likely to do more deep (4-10") vascular damage (which is what it's all about).

2.  Overpenetration, particularly through interior walls, is an undesirable thing.  This is on the assumption that "home defense" means there are other innocents in: 1. Your residence.  2.  The apartment next door with shared wall space.  3.  The house next door.  By overpenetration we mean penetration through walls by projectiles with high to medium wounding capacity after these penetrations.

3.  The smallest performing shot in 12ga that meets standards articulated in 1. above is #1 buck, and this ONLY in the case that the shot is hardened.  This is a hard load to come by.  Just stop even talking about anything smaller (particularly #4 buck) before you start.  Smaller loads are not stopping loads for violent offenders and never have been.

4.  #1 buck and larger are rounded projectiles that penetrate like crazy.  Hardened they also do not deform and this means they zip through interior walls with the greatest of ease.  This is bad because of 2. above.

5.  Handgun rounds because they are slower and more massive, do not break up when encountering interior walls and therefore tend to penetrate them while retaining a great deal of their mass and velocity and therefore wounding potential.  Handgun rounds have far LOWER wounding potential than .223 or 5.56 rounds before striking walls and far MORE than .223 or 5.56 after striking them.  This is exactly the reverse of the desired condition.

Poo poo the Ammo Oracle all you like.  Recognize, however, that just by the fact that you are having this argument brands you as a Johnny come lately in this subject.  We don't write passages in the Oracle because we are guessing.  We write them because the facts (like the lesser wounding of 5.56 after interior wall encounters) support them.  If we have our facts wrong (as we occasionally do) all one need to is correct us by sending me or Troy or Brouhaha an email with a citation to the correct information.  We rarely get taken up on this simple request.  This leads me to believe we tend to get it "right" "most of the time."  Again, feel free to correct us.  Until then you are engaging in ballistics mythmaking and armchair physics.  Not a noble profession.

 Something I've noted after taking several Handgun/Rifles classes:
 1.  Rifles, such as AR15, are advocated by most instructors for home defense nowadays.  Sure, AR15 and its ammo, .223  is not perfect in all aspects, but better than some other alternatives.
 2.  Rifles, such as AR15/.223 ammo, incapicitate the bad guy quicker because of the shock from velocity/energy dumping into the body.

 IIRC, there was a FBI data concerning about overpenetration issue, and it turns out that AR15/.223 ammo is more desirable than 9mm or shotgun.  Now, it's not perfect, just more desirable.
 There is one SWAT department in Metroplex area that I know replacing their MP5 with M16 Commando.
 
 My 2 cents...

Quoted: by the way, how, in the name of physics, does something LIGHTER, SOFTER, and SLOWER remain deadlier after wall penetration?

If you mean #00 or #1 buck it's because it doesn't break up or deform.  It therefore maintains its mass and velocity, the two key elements to lethality in moving projectiles.

Yeah, .223 might yaw, but that's a moot point.

With respect to any of the fragmenting .223/5.56 (OTM) it does far more than yaw.  It breaks up into small pieces.

I remember someone here doing wall-penetration tests and that's the best they got. not that 12ga was better by far, but this overpenetration this is way overblown. Brick outer wall WILL stop it, if that's how your house is built, if it's all wood, the rounds WILL be outside unless they hit something much denser in between.

This is all rehash.  FBI firearms labs did these tests ages ago, many many times.  You really don't want to be using a shotgun if you are worried about interior OR exterior wall penetration.  End of story.