As a pup it starts in the kitchen while they are eating. Banging pans and ignoring them while they are eating. Bang them together, drop them unexpectedly. Beat on them in the next room. Clap you hands unexpectedly. Make all kinds of noise and act like it is perfectly normal.
When they show a don’t care demeanor, introduce the gun from a distance, while having their mind on something else. Dead Bird they are playing with is a good diversion if they react at all you don’t.
