Dropped by
getting one
Pirate invasions are the only reliable route, and the odds off a rank and file pirate are bad enough that most players finish dozens of them without a sniff. Parrots are excluded outright. Pirate captains carry much better odds, and the Flying Dutchman is in a different league again, so the practical approach is to run invasions deliberately and make sure the Dutchman actually shows up and dies rather than letting the event wind down early. In Celebration Mk 10 worlds none of that applies, since the Princess simply sells it once you have cleared one invasion.
the gun has no damage of its own
Every point of damage comes out of the coin you load. Percentage bonuses multiply the weapon's own number, and that number is zero, so a damage accessory or a specialized armor bonus lands on the ammunition instead of on the gun. Your damage still climbs in practice, but the weapon itself never scales with anything. Critical strike chance and ammo conservation behave completely normally, which is why Shroomite and Vortex gear are still the right armor to wear with it and why an Ammo Box still pays off.
best modifier
It can only roll modifiers touching speed, velocity, critical strike chance and knockback, since there is no damage to modify. Agile is the one to want. The game labels Rapid or Hasty as its best, but that ranking is based on which reforge raises the sell value most, and it has nothing to do with how the thing performs.
your wallet is the magazine
Loaded with the top denomination it puts up numbers in Sniper Rifle territory with several times the trigger speed, and it is the most expensive way anyone has ever played this game. Copper is close to free per shot and does a fraction of that. Both modes are real. Copper is absurdly more damage per unit of currency, which makes it the correct ammunition for clearing trash, while platinum exists for the moment you want something dead now and do not care what it costs. Enemy defense also eats a larger share of a small shot, so the cheap rounds fall off harder against armored targets.
the stack management problem
Coins merge upward on their own when you pick them up. Let a low stack run dry mid fight and the gun quietly starts eating silver, then gold, then platinum, and you will not notice until the ammo counter has done real damage to your savings. Keep a deep buffer of small change crafted down in advance, watch the counter, and carry an Encumbering Stone if you want the floor to stop restocking your pockets while you shoot. Ammunition is pulled from the dedicated coin slots before the inventory, which is the sorting you want.
where it pays for itself
Pair it with a Lucky Coin, a Coin Ring or a Greedy Ring and a long fight starts refilling its own magazine, which is the only setup where the weapon is genuinely sustainable. Expert worlds help too, simply because everything drops more currency. Journey Mode breaks it outright, since duplicated coins cost nothing and the ammunition problem stops existing.