I think this is getting into semantics. If you think that bullets, by their nature, "are harmless," i would imagine that your definition of "harmless" includes far more things than most others' would - which is okay, but important to recognize that you're opinion probably isn't widely held. That's why regulations on bullets exist.
I understand your point of view and I recognize that it's consistent and rational, but our state operates functionally different than the one for which you're advocating. The fact is, our laws prohibit certain activities, that in and of themselves, do no harm to others, are easy to do, and are inconspicuous. Given your argument, I'd imagine you think that those activities (like the possession of heroin, for example) should be legal, but they're not.
Can you do anything legal and useful with heroin? I mean I think it's reasonable to think if someone is caught with heroin that their either going to use it (illegal) or sell it (illegal). Whereas millions of Americans drive around or carry everyday a loaded pistol, never with the intent of doing anything harmful or illegal.
Ok, now you're getting into something completely different than what we were talking about (harmlessness) and a little convoluted.
Citizens can do something useful with heroin: Inject it into their body and feel good.
Citizens can't do anything legal with heroin because...possession of it is already illegal. But at some point it wasn't illegal and in that period, people were going around using heroin legally.
Citizens can do something useful with ammunition: Stop a bad guy.
Citizens can do legal stuff with ammunition because...possession of "lethal" ammunition isn't illegal. But it would be if legislators made it illegal.