How do I change the drop/spawn rates?

Trying to find anything about drop rate/spawn rate of items. How do I change drop rate and spawn rate of items, different items from different mods etc? Would appreciate if anyone have any idea how to mod that. For example can I put a minigun on a feast but have it to be rare? Can I have the demolition pack (Gun pack) those guns spawn rarely across the map/from zombies?. Thanks! :D