IMO it's a lot simpler to just rig it in the Setup room, rather than the parent prop business.  Watch Phil Cooke's rigging tutorial, think about it, and give it a try, I bet you'll figure it out very quickly.  You only need two polygon groups, one group for the gun and ammo, and one group for the tripod; and two bones, one for the gun/ammo, and one for the tripod.  Turn off "Bend" property for the gun bone.

http://www.philc.net/rigging1.php

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