The problem is in the tech, you see it in Red Orchestra as well.
When you press the mouse button, triggering the weapon, there is also a trigger attached to the animation system which tells the game to play the firing animation. That animation is a child of another animation state which in BF2's case is the standing animation. So, when you trigger the fire animation, the game then goes back up the tree to the stand animation and then onto the next trigger, which would be the "shift" animation where he cycles the bolt.
In between, that piece in the back is bouncing back and forth and doesn't stay forward, because it is in different locations for each animation.
