@Cassity14's question got me thinking so I decided to do a little more research. I don't really understand what the definition of an "assault weapon" is, because both the AR15 and many handguns are semi-automatic. From my brief research it kinda seems like a made up term that doesn't have a solid definition. For the purpose of this I'll be basing it on how the guns look. If it looks like a handgun, I'll call it a handgun. If it looks like an assault rifle I'll refer to it as an assault weapon. If you're knowledgeable on guns feel free to correct me.
@gourimoko @King Stannis @The Human Q-Tip
The CBS article I linked earlier describes the 18 most deadly mass shootings in US history. I wanted to look and see the type of guns used in each.
Las Vegas, Nevada - Route 91 Festival - 58 dead
Assault weapons
Orlando, Florida - Pulse nightclub - 49 dead
Assault weapons
Blacksburg, Virginia - Virginia Tech - 32 dead
Handguns
Newtown, Connecticut - Sandy Hook Elementary School - 27 dead
Assault weapons
Sutherland Springs, Texas - First Baptist Church - 26 dead
Assault weapons
Killeen, Texas - Luby's Cafeteria - 23 dead
Handguns
San Ysidro, California - McDonald's - 21 dead
Handguns and shotgun
Parkland, Florida - Marjory Stoneman Douglas High School - 17 dead
Assault weapons
Austin, Texas - University of Texas - 16 dead
Had a whole slew of weapons. Sniper, M1 Carbine, shotgun, handgun, knife
San Bernardino, California - 14 dead
Assault weapons
Edmond, Oklahoma - 14 dead
Handguns
Fort Hood, Texas - 13 dead
Handguns
Littleton, Colorado - Columbine High School - 13 dead
Bunch of guns. Shotgun, sawed off shotgun, handgun, hi-point carbine
Binghamton, New York - 13 dead
Handguns
Wilkes-Barre, Pennsylvania - 13 dead
Assault weapons
Camden, New Jersey - 13 dead
Handgun
Aurora, Colorado - Movie theater - 12 dead
Assaults weapons
Washington, D.C. - Navy Yard - 12 dead
Shotgun and handgun
It appears that a majority of the 18 shootings involved "assault weapons", but I think this shows that people armed with only handguns are more than capable of carrying out mass shooting attacks.