For a very concealable backup pocket gun I would go with something like an LCP or a hammerless lightweight revolver.
It has been mentioned here a revolver doesn't even need to be drawn to shoot it. I really "DON'T" suggest shooting a revolver while still inside your pocket. If you do then expect your leg to be hurting and more than likely bleeding. Expect your pants to have a hole blown through them. Goodness sake you better hope your scrotum isn't ripped apart.
Check out the hickok45 video how not to shoot a revolver or check out Myth Busters video on revolver dangers. I carry an LCR 357 in my front pocket and there is no way I am going to fire it while inside my pocket.