Shoot any soldier (but preferably one with a rifle and another weapon) then make various attempts to loot his two weapons and other gear.
It can also be reproduced by placing the player in the editor, placing a soldier immediately in front, previewing the mission and shooting him.
As long as a 'take weapon' option is shown in the action menu, you cannot pick up magazines or other gear from the body. (Situation with an AT launcher illustrated in attached screenshots.)