Note: You are just holding your breath, that is all the hold breath key does. Your arm muscles are still relaxed. To hold them steady you must move your mouse and aim the gun.
Comparing this (Not held breath) http://a.pomf.se/agynfx.webm
to this (Held breath) http://a.pomf.se/fcnpxk.webm
There is a massive difference, and that is without even attempting to hold your arms steady.