Minishark

The Arms Dealer sells the fastest gun in the first half of the game, and the price is steep enough that buying it usually means skipping something else. It is worth it. The fire rate is high enough that a stack of the cheapest bullets clears a Blood Moon crowd or a corridor of Dungeon skeletons without much aiming, and a share of the shots cost you no ammo at all, so the drain is far lower than the trigger speed suggests.

Defense subtraction is the problem. Every bullet is small, so anything heavily armored eats almost all of it and each hit lands for the minimum. Against bosses it can be close to useless on its own. Armor penetration from a Shark Tooth Necklace pulls it back into usable territory, and picking a stronger bullet matters far more here than it does on a slow gun. Fossil armor pushes the ammo saving higher if you plan to hold the trigger for long stretches.

• It has no knockback at all, which locks out every modifier tied to knockback. Demonic is the best roll left to it.
• Buying one and selling it straight back before you close the shop window unlocks an achievement and costs nothing.
• Keep it. It is the base of the Megashark, and the fins for that upgrade are the slow part of the plan.