Jon - Thanks for the info.
I guess my take on this issue is this - IF the 5.56 bullet fragments, then it will have less penetration through interior walls in the event of a miss. However, nothing works 100% of the time. I just don't trust that a 5.56 that misses will fragment upon hitting a sheetrock wall.
OTOH, a buckshot pellet will ALWAYS have less velocity and sectional density that a 5.56 rifle bullet. And in most cases, the pellet will also have less weight (especially if you are using #1 or #4 buck) and less momentum.
So, do you trust the 5.56 to fragment if you miss? Or do you go with what you know will always have less velocity, weight, and sectional density? No right answer, but that's the choice.
For home defense at close range, I'm still sticking with my shotgun. For patrol use, especially when you consider the possibility of body armor, I can see why more police agencies are going with a rifle.
One quick point about the body-armor-wearing bank robbers mentioned earlier...I saw some video from that shootout. It is true that one of the officers fired a shotgun at one of the robbers. However, he failed to immediately fire another shell at the robber's head. Instead, he fired once, and then stopped to see what would happen. What happened, is that he got pinned down by a hail of full-auto AK fire after his buckshot failed to penetrate the robber's multiple ballistic vests.
The lesson there is that no matter the weapon, fire COM, and then immediately target the head if the BG does not cease his activity. At the distance he was at, the officer would have had a good chance of multiple pellets striking the robber in the head, had he immediately taken the shot. Another situation where the equipment was blamed, rather than tactics and training. (No offense toward those involved - just pointing some things out for "lessons learned.")