Yes, you are correct. Most mass shootings use handguns. However....the deadliest shootings use rifles like the AR-15. The difference between being shot by a 9mm bullet from a Glock compared to the usual .223 round used in an AR-15 is night and day. Yes, the handgun bullet is larger...but it doesn't have the same amount of propulsion in the cartridge. The AR bullet goes like three times the speed and causes incredible damage when it strikes the body. It tumbles inside destroying organs and blood vessels, leaving massive exit wounds. When the rifle was being developed in the early days of the Vietnam war, there were reports of a VC solider getting hit in the butt, and bleeding to death really quickly due to the massive and traumatic damage it did to his body.
The rapper 50 Cent famously got show like 9 times and survived. He was hit by a handgun. Yes, it did permanent damage to him, but it was no way comparable to taking one round from a high powered rifle.
The Washington Post just did a whole thing on the damaging power and difference between these types of firearms, showing exactly what the AR-15 does to victims:
Eliminating AR-15s would not stop mass shootings. However, it would likely make them less deadly...