Not really sure what you mean by "high caliber".  The AR-15 has smaller caliber, with higher speeds.  Most guns are semi-automatic these days.  
The AR-15 was designed as a military rifle.  It has a high rate of sustained, accurate fire.  They say that the AK-47 is the best gun if you want to kill everyone else in a room.  Well, the AR-15/M-16 is really good if you want to kill almost everyone else in a room, while sparing one or two specific ones.
I would argue that 9mm stacked magazine pistols are more lethal in America.  They are more easily carried.  You have to know that you plan to go on a killing spree to carry an AR-15.