you know, those forearm mounted guns you see in sci-fi would probably need some kind of safety device to stop them from firing if your hand is in the wrong position so you can accidentally blow your own fingers off, they kind of put a lot of delicate body parts in front of the muzzle
you can only be so cool with your pop up cyber gun concealed in your forearm if you end up accidentally shooting your cyber hand and shattering it into lots of little pieces
Consider you can utterly ruin some coroner's day when they have to write, 'subject killed by high velocity finger bang.'