You'd lose that bet. Statistically, handguns are more likely to be used in a mass shooting than a rifle. Off the top of my head, the Giffords shooting and Charleston Church shooting used handguns. It's not like if you're shot with an AR at close range you're dead, but if you're shot with a .45 at close range you rub some dirt on it and walk it off. If the shooting is taking place at close range, a handgun will kill just as many as a rifle.
Anybody can go Into a church or public place and kill people. Charleston had what 9 people die? That doesn't really make your point.
Vtech was a better example but like I said dude was loose for 3 hours. What do you expect? Him to stop and have a picnic? Authorities failed and he had a ton of time and ammo to use