Golden Bullet

The point is the gold marking it leaves on whatever you hit, which makes that enemy cough up noticeably more coins when it dies. It is not a damage upgrade, so loading these for a boss fight is a waste of good bullets. Save them for events and farms, where the coins are the actual reward you are there for.

What matters is value per round, so slow heavy guns are the right home: one expensive shot, one dead enemy, full coin bonus. Anything that sprays chews through your stock to apply a mark the first hit already applied. The exception is the guns that throw several projectiles off a single round, since those get the wide coverage for free.

Everything that goes into them is bought from NPCs rather than mined, so a large stack costs shop trips and nothing else. That is what makes them practical for a long grinding session: you can restock indefinitely without leaving your base.