He used four handguns. He did have an AR-15 but it was left in his car. He tried to buy one at a gun store and was refused, so he murdered his mother and took hers. But he didn’t use it.
This is a video of a news report aired on MSNBC on December 15, 2012. Lanza was able to shoot a lot of people because it was a gun-free zone and no one there had a gun with which they might have stopped him. The police arrived 20 minutes after the first call went out. Lanza had free reign to shoot as many people as he wanted in that time, he didn’t need any special type of firearm. If schools were not gun free zones there might have been someone there with a CCW permit and a gun, and Lanza might have been stopped. He also might never have gone there in the first place if it had not been a gun-free zone. Connecticut issues CCW permits to citizens with a clean criminal record and no history of mental illness, but they cannot carry in gun-free zones.
Seung-Hui Cho, the Virginia Tech gunman, also did NOT use an AR-15. He killed 32 of his fellow classmates with two handguns, a .22 caliber Walther P22 and a 9mm Glock 19. He did have both 10 and 15 round magazines, but if he’d had only 10-round magazines would that have made any difference? Like Adam Lanza, he had complete free reign to kill his victims because they were defenseless. The Virginia legislature had previously tried to give schools like Virginia Tech the ability to allow those with CCW permits to carry on school grounds, but it was defeated. Virginia Tech was a leader in defeating this proposed legislation.