The primitive firearms that the Klingons secretly provide to one faction on the planet Neural, leading Kirk to arm the hill people with the same weapons.
Flintlock muskets are at the centre of the original series episode A Private Little War. Kirk remembers the planet Neural as a peaceful place whose people hunted with bows, but on returning he finds the villagers shooting at the hill people with guns.
Such a leap in technology cannot have happened naturally in so few years. Kirk and McCoy find a forge in the village where the Klingon agent Krell is supplying the leader Apella with parts and promising improved models.
Kirk decides that the only way to preserve both sides is to give his friend Tyree's hill people exactly equal weapons. McCoy protests that this is a path to endless war. After Tyree's wife Nona is killed, Kirk asks Scott to make a hundred flintlocks, which he bitterly calls serpents for the Garden of Eden.