Revolver

This is a rhythm weapon. Tap the fire button right as the gun finishes its previous shot and it winds up, firing progressively faster and critting far more often. Hold the button down and autofire takes over, which switches the whole mechanic off. Sloppy timing, or just standing there holding it without shooting, bleeds the crit bonus back down again.

At full wind-up it roughly doubles its own rate of fire, which turns a merchant purchase into a genuinely strong pre-hardmode gun. Getting there takes attention you might rather spend on dodging. If clicking to a beat during a boss fight sounds like work, almost any other gun will do more for you in practice. If you can hold the pattern, nothing else at this stage keeps up.

The Traveling Merchant only offers it once you have cracked open a Shadow Orb or Crimson Heart, and he is expensive, so it usually appears well before you can afford it. He also carries it as his own defense weapon, which is a nice touch until hardmode arrives and he upgrades.

Loaded with High Velocity Bullets it fires about as fast as anything in Terraria, which means almost no lead time on a moving target. For long range plinking that matters more than the damage does.