I keep my 870 'cruiser ready'. Always have...same as my Bushmaster. Might slow me down some, but I ain't too worried about it.
For many with LEO backgrounds, it's the way we are used to carrying longarms. When you pull the M4 or shtogun from the rack, you rack a round into the chamber and know that the safety is off already. I think it came mostly from police administrators not wanting to have the shotguns with a hot chamber (maybe there were a few too many holes in the top of the CV's) and also a general dislike for safeties and some agencies had an assortment of shotguns (Remmies, Winnies, Mossies, Ithacas etc.) with safeties in different positions.
So many police departments required all the shotguns to be stored "cruiser ready" with the hammer down on an empty chamber, with the tube full, and the safety off. As carbines came back into vogue, most of the trainers taught the same method for the rifles.
I would never even consider a debate between the two. I have seen a man eat 3x5.56 rounds center mass and multiple other hits (later as he was running) and still run and run...... It is highly ineffective caliber against perpetrators as its velocity is so much and circumference of bullet so little that it passes straight through human tissue.
I've seen men eat up multiple rounds of 7.62x51 NATO, 7.62x39, .45 ACP (and other rounds), and multiple shotgun hits (though I don't remember seeing a buckshot wound). What's your point? BTW, that doesn't mean that I don't consider all the above as highly effective rounds. However, if you punch through the chest without hitting the heart, a major vessel, or the spinal cord, they just aren't going down (or going down quickly). Even pneumothoraces (dropped lungs) aren't instant manstoppers. It's as simple as that.
I must also say that I have never seen a 5.56 center mass that didn't cause a fairly massive exit wound (due to it's tumbling more quickly than a 7.62--which tumble also but it takes more tissue). Since your "experience" is so different than my own (and trauma photos and ballistic gelatin testing), I have some doubt that you have seen as many wounds as you claim. Perhaps your "poodle shooter' comment should tell me that you get your opinions from a certain dead gunrag writer.
A 12-guage shot gun will put down any human (or animal on the face of North America with the right load) if placed center mass. There are no doubts about a full once + of lead in a grouping the size of your fist making chunky chicken out of its target...were talking lethality.
On this we agree. 12g with buckshot is HIGHLY effective. I carry one occasionally at work (mostly at night or for special purposes) and have one as my primary home defense weapon (mostly because I cannot justify a home AR). However, I grab an AR at work most of the time. I just prefer the precision, lighter weight, and better firepower. I certainly would not feel at a disadvantage defending my home with one.
I wouldn't ever buy a pistol caliber carbine, unless I get a real good deal on a .357 or .44 lever gun with a 16" barrel.
Just because I want to.