Musket Ball

This is the base every other bullet is built from, so buying far more than you think you need is always right. The Arms Dealer sells them in bulk cheaply enough that ammunition stops being something you think about, and every recipe that upgrades a bullet starts here.

Several guns rewrite whatever you feed them. A stack of plain rounds loaded into an Uzi comes out the barrel as something much faster, and a few weapons convert any bullet at all. Check what your gun does with ammunition before you spend gold on the expensive stuff, because in some hands the cheap rounds are already the good rounds.

For clearing caves, killing an event crowd or anything you can afford to be sloppy about, plain rounds hold up well into Hardmode, and the Endless Musket Pouch removes the last reason to think about it. Bosses are the exception. Chlorophyte or Crystal rounds shorten those fights enough that the cost of making them is irrelevant.