How to crush dry foods into powders (or at least smaller grains)?

Quick question to the more experienced cooks out there. I'm currently making high calorie homemade protein bars, and while the amount of food in there in terms of grams is reasonable, some of the ingredients like oats and granola take up more space that others which would make the bar inherently larger. I would like to crush these ingredients up so they take up less space, but was wondering if there is a tool that would be used to do this better than just me with a large surfaced 'flat hammer'. Whether it's an appliance or hand tool, all answers are welcomed as I'm still in the final stages of the logistics. Thanks in advance.