If you claim that guns are the reason for mass killings, you are ignoring that the largest mass murders in the USA did not use a gun. Here are a few of them:
1 On 911, there were 2,996 dead and over 6,000 injured. The weapons used were box cutters.
2 The Oklahoma City Bombing. There were 168 dead and over 680 injured. The weapon used was a bomb made from fertilizer and diesel fuel.
3 The Bath School massacre of 1927: 45 dead, and 58 injured. The weapon used were a pair of bombs.
4 New York Bombing of September 1920: There were 40 killed and several hundred injured. The weapon was a bomb.
5 The Upstairs Lounge Fire in New Orleans on June 24, 1973 was an arson that was intended to kill the patrons of a gay bar. It worked, killing 32 and injuring dozens.
6 The LA times is attacked with a bomb in 1910, killing 20.
Columbine and Sandy Hook don't even make the top ten.
The fact is that guns have nothing to do with spree killings. The first mass murder in US history happened in 1780, and was committed by a man named Barnett Davenport. He killed an entire family (all 5 of them) during a pre-planned burglary. He used blunt instruments to beat the adults and one of the children to death, and then set the house on fire to kill the other children.