With any unit that has ammo (Archers, Javelins, Throwing Knives, etc), wait until you are out of ammo. Press X to back out of the unit, and wait for the AI to shoot of their own accord. Re-join the group, and you should have about 10 more ammo, which is not a full restore. When you run out, just leave the squad, wait, and re-join. It can be done infinite times.