'Mass shooting' is defined as 'shooting three or more people', so yeah, pistols are used far more than rifles.
Go to YouTube and enter this in the search bar- never heard about any rifles and the crowd would have freaked out if they had seen any.
'17 people shot on N. Water St. Friday night, Milwaukee police say'
Also,
Handguns are the most common weapons used in mass shootings in the United States. Other types of guns used in mass shootings include rifles and shotguns.
www.statista.com