My observations as far as scope movement:
<b>Prone--</b>almost completely still
<b>Kneeling(<i>unsupported</i>)--</b>a little movement, but not enough to throw me off target
<b>Kneeling(<i>supported</i>*)--</b>very little momement
<b>Standing(<i>unsupported</i>)--</b>a little more movement than kneeling and slightly harder to keep on target.
<b>Standing</b>(<i>supported</i>*)--</b>very little movement
* <i>supported</i> means braced against a solid object like a wall, a tree, a rock, et cetera.
As far as the breathing affecting weapon movement is concerned, it is all about muscle control. If you slow your breating the movement becomes almost negligable, IMO.
Also, I have never targeted an object more than 300m out, so that may be what is effecting experience and subsequent opinion. My experience with scoped weapons is also limited only to rifles. I have never shot a scoped handgun.