With any unit that has ammo (Archers, Javelins, Throwing Knives etc.), wait 
until you are
out of said 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 ~10 more ammo (not a full 
restore).
When you run out, just leave the squad, wait, and re-join. It can be done 
infinitely.