In the immediate aftermath of the shootings, such reports were circulating, but they were incorrect.
According to
this report from the Connecticut Post, CT state police have said that the Bushmaster recovered from inside the school was the weapon used to kill all the victims. Lanza's only use of a handgun was to shoot himself.
This is based on the reports from autopsies done on the victims. There were earlier reports that reflected confusion about where various weapons were found, but the autopsy reports are definitive.