But if you truly wanted to kill people dead without question, there are MANY rifles out there in calibers that would be a far better choice than the AR15 chambered in .223.  Good grief, the Winchester Model 1894--yes 1894--in caliber .30-30, is a lever action, but a lever action can be fired extremely fast by a practiced shooter.  And the .30-30 will tear you up at close range, especially the soft point ammo more often associated with the cartridge, rather than the less lethal full-metal jacket ammo normally used in the AR15.  But that's just one example.  There are so many rifles chambered for much larger, more powerful cartridges than the .223, and they make the .223 look like a pipsqueek. 

This whole idea about the round used in the AR15 is liberal politician and journalist's misconception.  They've glommed onto something to hate because they know nothing about firearms, and because the AR15 is so poular that its something they can remember in their talking points.  Have you ever seen a human being shot up close with a .30-06?  Look up some photos of WWII.  And yes, the .30-06 is still one of the most popular cartridges in America today.  If you ever read anything but leftist media.

"A shot (shockwave) froman AR15 does serous damage to organs within a few inches of the trajectory?"  What you mean is the phenomenon exibited by all rifle bullets which strike the target at velocities at greater than about 2200 feet per second, and which is called hydrostatic shock.  As the name implies, it is a wave that ripples out from the immediate damage zone and can cause additional damage to the target, although in general it is believed that this is not a serious additional damaging effect in targets larger than whitetail deer, which usually are lighter than an adult male human.  The real damage to a target however is caused by the bullet itself.  The .223, most often an FMJ in criminal shootings, is quite small, and the damage is not great--the US military has been disatisfied with the .223 for years, but is restricted to FMJ use.  US Special Forces first complained about the .223 in Iraq, and eventually the military dropped the .223 for a larger caliber (diameter) bullet that should do more damage.  They are in the process of changing over.

Why is the AR15 the weapon of choice of mass shooters? Maybe because it easily kills or maims?

Hopefully I've pointed out that this this comment above is an utter falsehood.