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Wrist mounted...

I need some help. I need ideas on how I would make this. Basically I want to make a wrist-mounted pistol (fake).   If you've seen Assassins creed before, they use a hidden blade that is spring loaded. Also this was in Sherlock Holmes (professor). What I want is is to sit on my wrist and when I push a button behind the pistol, it ejects it forward so i can grab it.


I apologize if I'm not being clear.

Picture of Wrist mounted...