For some context, I'm planning on running a D20 Modern game, using the D20 Apocalypse supplement. One of my players wants to play a bullet merchant, so they'd be crafting and selling bullets. I've looked through both D20 Modern and D20 Apocalypse, but I cannot find anything about how to craft bullets. Are there any official rules on crafting ammunition? If not, does anyone know how it would be done within the existing rules?
You're in luck - there are indeed official rules for ammo crafting in the very D20 Apocalypse sourcebook you're reading. They are, however, somewhat scattered about the book which makes tracking this down a little on the tricky side.
Crafting ammunition is a special function of the Craft(mechanical) skill described on Page 58, and generates up to 10 bullets as a moderate craft task consuming shell casings and mechanical parts, as well as requiring an ammunition kit to not take an improvised tools penalty.
Based on the repairs and crafting table on Page 30, this makes the task of crafting bullets a DC 25 craft check, and consumes 5 units of Mechanical Parts in addition to the necessary 10 shell casings. Somewhat on the tricky side, but not outside the reach of our prospective bullet farmer - the minimum expenditure of 5 parts alone provides a natural +5 to the check, and with some investment in Intelligence and the Craft(mechanical) skill this should be within reach for even a first level character with effort.