Uzi

Angry Trappers live in the hardmode Jungle and hand this over rarely enough that nobody plans a run around it. When it does show up it arrives before the mechanical bosses, which is the whole appeal: a fast, accurate gun at a point where most players are still swinging an early hardmode sword. At launch the drop chance was so absurd that almost nobody found one, and it was corrected within a patch.

Feed it the cheapest bullets you own and they come out as High Velocity Bullets. The conversion does not raise the damage figure, it changes the flight: shots cross the screen faster and reach further before expiring. That means almost no leading on moving targets, and it is one of the only ways to shoot High Velocity Bullets before you can make them.

Against the Megashark it loses on raw output, but it costs no boss fight and no Illegal Gun Parts. Treat it as a bridge weapon that carries you through the awkward stretch after the Wall of Flesh. Once the mechanical bosses are down, the Megashark line takes the slot back.