That term is just for show and talking points. If you want to evaluate the "effectiveness" of a weapon you would not choose an AR for the weapon of choice. Did it work on defenceless children? Yes, unfortunately.
Who the hell cares? You may not call an AR-15 an "assault weapon" but the people who want to take them away sure as hell do! Whining about "show and talking points" isn't going to be worth squat when these things wind up on a banned list. An AR-15 isn't an assault weapon? Whatever. Tell it to the judge, if Feinstein gets her way, as he's handing you 10 years in the joint for possessing one.
If you want to evaluate the "effectiveness" of a weapon you would not choose an AR for the weapon of choice.
If you want to kill as many people as possible and you have two pistols, a shotgun and an AR-15 with several loaded magazines, which weapon are you going to use? Newsflash: The .223 round has killed more people than atomic bombs. It's plenty effective. A million dead Vietnamese would tell you if they could.