It may not be much for multiple shots, but my favorite "harvesters" are my blackpowder muskets and rifles. Flintlocks aren't the most reliable, but they are relatively maintenance free besides chipping new flints and occasionally rehardening the frizzen. Your ammunition, lead, is one of the most common elements and is 100% recyclable. Finally, the propellant. Sure, you could buy factory blackpowder, but it isn't until you know the recipe yourself until you are truly liberated. The ingredients are simple: soft wood to make into charcoal, sulfur (found naturally or in many anti-fungal foot powders), and a chicken yard (for leeching your saltpeter-- or just get it from a drug store).
Once knowledge is known, it cannot be taken away.