Yes. All the children were killed with .223 rounds. Lanza wanted to kill as many people as possible. Why would he have left the most effective weapon he had at his disposal back in the trunk of his car? The weapon left in the trunk was a shotgun.
As much as I wish this wasn't an "assault weapon" related crime, fact is, it was, and all the conspiracy nuttery isn't going to change this.
I’ve changed my mind and I’m withholding judgement on what gun(s) were used by Lanza until I see an official report. Too many lies were spewed in a very short time.
http://www.digitaljournal.com/article/340113
See Pete Williams report. It contradicts the story that was promulgated later in the above link and here below.
http://www.youtube.com/watch?v=IqpjJ6P-NSQ&feature=player_embedded#t=0s
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.