I'm always leery about subverting the players' intentions. If the player is choosing to make Guns based attacks, that player clearly wants to inflect Stress and eventually Take Out the target (or perhaps stop at an acceptable Concession). I might offer to change the action to a Maneuver or the like, but I'd never just do it. That strikes me as disrespectful of the player's right to make their own decisions.
I certainly appreciate that - there is one textual example, though, when Harry and Billy are chatting in the sidebar, and Harry complains about a Fists attack that only hits, but not enough to cause Stress. Billy suggests that it instead be retroactively converted to a Maneuver.
Perhaps a compromise would be something like "You hit, but deal no damage. May I offer you the option to convert your action into a Maneuver? It's not getting hurt, but you could opt to be 'Distracting' it, or 'Drawing its Attention to You'?"