Properly placing hands on the gun while reloading.

I'm creating a uncharted style 3rd person shooter using mocap animations. While reloading right hand of the character always stays with the machine gun while left hand simply changes the magazine. So I linked right hand to the MG using a multi parent constraint from new animation rigging package. But left hand is not alligned to the magazine while it is pull out and insert back. (I'm using humanoid rigs)

1)So is there a way to do it within unity?

2.If not can I do it within Motion Builder? Then how can I put my gun to the same position and rotation in both programs?

Thank you. I tried different things for weeks but could not come up with a solution. also I couldn't find any detailed documentation about new animation rigging package.