ok, so you people are trying to change the location within the model of the model bone... during the chor? No, I don't think that can be done. You can only move the model bone in the model window and that isn't editable from within the chor.
However, (and maybe this had been mentioned above already, i'm not sure) all you need to do for the same effect is add a null to the chor, place it at the place you wanted the model bone to originate from and then translate and orient constrain the model bone to that null. New pivot point! Animate with the null instead of the model bone.
Bonus: you can vary the location of the model relative to the null for different situations.
For example: model bone pivots near tip of gun, then Null pivots near the trigger, but say later in the shot the rifle gets dropped, and lands on the butt before falling to the ground. Move the null pivot to the end of the rifle butt and it is easy to animate the rifle falling over with the butt staying where it landed.
riflepivot.mov
riflePivot02.zip