It just so happens that I'm making a marine map right now also. There are several methods you can use for this:
Method #1: Make a new custom ability based off of Armor +1 or something. Set the field that gives armor to 0. In the Art - Target field, select what weapon you want. Make a string in the Art - Target Attachment Point field with 'weapon' (without the quotes). Finally, give the ability to your unit.
Method #2: Make a trigger that has the Create Special Effect On Unit action. Pick your weapon model, and type 'weapon' in the attachment point (labeled 'overhead' by default).