I found a random youtube channel a while ago where they replace buck shot with random objects and see how they fire and how accurate they are. Im certain I seen curving on irregular objects. Imagine a bullet made up of two different materials with one half being heavier than the other. If you shot it from a rifled barrel it would spin (and destroy the barrel probably) but once it left the barrel it would begin to spin on a different axis and start curving. Well I think so anyway.