If you do that, then your gun will appear in the reflections as if it were painted on the far wall.
The fundamental problem is that in order to draw reflections of things behind the gun, you need to actually render the things behind the gun. So the only really general solution is to fully render the scene without the gun, then render the gun separately and composite it onto the screen as a post process after screen space reflections are computed. You can do this with scene captures and render targets.