If the guy pulled people over by pretending to be a cop and then executed them in their cars I would assume it was a handgun. Because who has ever seen a traffic cop walk up carrying an M-16?
He started with an "assault duck-hunting shotgun", then offed the policewoman, acquiring her pistol and long gun.
I presume the long gun was some sort of semi-auto.
Update: 22 PEOPLE KILLED IN NOVA SCOTIA RAMPAGE
April 21, 2020
https://www.freerepublic.com/focus/f-news/3837746/posts