Ammunition not natural?

Lead is a rather common element. I can make potassium nitrate (saltpeter) in my back yard from old shit (literally, old shit) and sulfur is found in the volcanic remains near here. Charcoal can also be made, and willow (which grows in abundance) makes the best for gunpowder.

Nitrocellulose is simply nitrated cotton (other cellulose could be used, of course). Nitric and sulfuric acids are also made rather easily.

As for ignition, flint and steel (both are, ultimately, common).

Or, more simply, the archers at Agincourt set a pretty good pace in loosing arrows, didn't they? Silent, and an arrow will quite likely pierce a "bulletproof" vest. At a hundred yards or a thousand, you're still stuck.