"Someone please explain to me what is so special about the #AR15 (style) assault rifle(s) that can't be done with the myriad of shotguns, hunting rifles, revolvers and semi-automatic handguns that have been available for decades?" Tl #1
rj #4 is half right, wrong as much by omission as commission.
I've never owned an AR, but I've handled them. I qualified Marksman with the M16-A1 and am likely still proficient with it.
To address your question directly Tl #1, there are numerous differences, even if seemingly subtle to the layman.
a) The AR & the M16-A1 are carbines. This makes them substantially better suited to close-quarter combat. A long gun, a true rifle may be more accurate due to longer, and potentially more rigid barrel. Such long guns are suited to poised, sniper-style shots: one shot / one kill. But the AR & M16-A1 is better suited for shorter range (up to ~250 meters) rapid fire.
b) A 12 ga. magnum slug is heavy. The .223 Rem. is far lighter, and thus pound for pound the ammunition the AR & M16-A1 fires may provide a 4:1 advantage.
c) Rate of fire.
The M16-A1 is capable of auto-fire: one sustained trigger pull empties the magazine, 20 rnd in ~2.2 seconds.
The AR fires semi-auto. One trigger pull, one shot fired, until the magazine is exhausted.
How many school children were massacred at Sandy Hook? A dozen?
A psychopath could kill a dozen children with a muzzle loader. Maximum rate of fire ~1 rnd each 45 seconds. That's 9 minutes. 9 minutes is enough time for a COP to finish his doughnut, take one last sip of coffee, stroll over to the patrol vehicle, drive to the school, walk inside, and bop the shooter on the bean with a 2 x 4.
IN VIVID CONTRAST
With an AR (or M16-A1) a dozen humans can be shot dead in less than 10 seconds.
Get the picture?
I'm a licensed hunter.
For varmints I use a .22
I have no use for an AR.
"An AR-15 is a civilian version of a "battlefield weapon" and it is perfect for self-defense" BD #5
In what scenario?
For home defense?
3 thugs cornering you in a dark alley?
Keep you AR for such scenarios. I'll take a Colt .45 (M1911-A1) for that.