Unless an armor is full covering, always respect even the smallest and weakest projectile. And even if it covers the entire body and head, still be careful.
Then, there's the problem of spears and arrows: they may have less than a tenth the speed of a bullet, even when thrown by a proficient user, but they do tend to have way more than ten times their mass. Whether a projectile can penetrate an armor is a matter of inertia, a combination of both mass and speed, not just a matter of either one of those two.