Short with procedural muzzle flashes created with Bang by time000666. Also check out the japanese language tutorial on how they were created.

The one and only Ryan Connolly from Film Riot shows how you can use Bang to very quickly add super realistic muzzle flashes to your action shots.

French action vlogger Atom has a great overview of Bang and how it can greatly clean up your comps and dramatically improve your muzzle flash workflow. It's in french but the auto-translated youtube captions are good enough to let you follow along.