Hitting a person in the head with a piece of metal can kill them. If you kill a person by hitting him in the head with a pistol, it had better be unloaded or you will almost certainly go to jail.
The use of deadly force is nearly always predicated upon the belief by the person using deadly force that deadly force is the ONLY reasonable option and that it is IMMEDIATELY necessary.
Hitting a person with a loaded pistol may be seen as an admission by you that deadly force wasn't warranted. Given the fact that YOUR belief about the necessity and urgency of the situation has a bearing on whether or not deadly force is justified, you may have just testified with your actions that you didn't believe it was necessary.
And hitting a person with your fist hard enough to knock him out may also cause permanent injury or death. It's not like the movies.
The idea that you can just tap a person, knock them out and wait for the cops to show up is a fairy tale.