I really think there should be a chance for weapons merchants to have bullet and shell casings in stock that you could buy in bulk, maybe at a hyper inflated buying price to prevent you from easily crafting mountains of the best ammo. As of right now, if I want more .44 explosive rounds, I have to just sit there and waste all my regular ammo shooting rocks until I have enough cases. Not only is this boring, but it doesn't really provide additional challenge, it just means you have to waste a bunch of time doing a repetitive task.
It'd be great to have a way of buying or crafting empty casings and shells to alleviate the boredom of shooting rocks all day.