"But the AR-15 style rifle — the most popular rifle in America, according to gun dealers —
was also the weapon of choice for Adam Lanza, who the police said used one made by Bushmaster on Friday to kill 20 young children and six adults in an elementary school in Newtown, Conn., in a massacre that has horrified the nation.
The increasing appearance of the rifle in rampage killings — an AR-15 was used by James E. Holmes, who is accused of opening fire and killing 12 people in a movie theater in Colorado in July, police officials say, and by Jacob Roberts, who shot and killed two people and then took his own life in a shopping mall last week near Portland, Ore. — has rekindled the debate about its availability and its appeal to killers bent on mass slaughter."
http://www.nytimes.com/2012/12/17/u...yle-rifle-in-newtown.html?pagewanted=all&_r=0
Given that Lanza's mother was 52 when her son killed her, she would have had to have been younger than 18 to have bought it before the 1994 ban. I think you would agree that is very unlikely.
Depending on the month she was born she would have been about 33 years old in 1994. Well old enough to purchase the rifle. Since she was a collector it is also likely she was interested in that rifle during that time.
Shootings with pistols do not usually have the high numbers of kills that are usual by guns with high capacity magazines.