the bushmaster ar-15 as well as a majority of other AR-type rifles are available in .223 AND .22(LR). Whether the one used in Newtown CT was a .223 or a .22LR is really a big unknown. I have heard the reports over and over and over, and I have not heard anyone mention the actual caliber/ammunition that specific gun used. personally I think it was a 22LR, NOT a .223/5.56-NATO
another thing to note: nearly ALL of the initial reports on Newtown, sourced from multiple first-responders of all varying types, stated the shooter was carrying and used 2 handguns, and that a semi-automatic rifle was found in the car he drove to the school. This story was repeated over and over by first responders, and second responders, and investigators, and various other people. It wasn't until 24 hours later that reports came out stating the shooter used an "assault rifle" on all of his victims, and that he had 2 pistols on him but only used 1 of them to take his own life.
sounds extremely suspicious to me.