its a 2 second clip rendered as a .gif
I was considering animating the casings ejecting from the gun but I thought that'd be visually too cluttered since there's already quite a lot going on for those 2 seconds and it'd draw too much attentions on the nonsensical/comical "flight pattern" of the casings i.e. it'd ruin the "joke"

just to share some reasonings, that might be of interest