Once you have almost used up your sniper rifle, beam rifle, or rocket launcher (with 1 shot left ), trade with your squadmate. Once they have that weapon, they have limitless ammo for it and can help you out immensely with the more powerful weapons. There are two points to note: if you try and trade your weapon back from them, it will still have the same amount of ammo that it had when you gave it to them and the energy sword cannot be traded with human squadmates.